I made the switch many years ago because the band needed a bass player, and we had a guitarist who was much better than I was. I am a decent singer - sang some lead vocals and some harmony - there was another guitarist who sang well and just played chords.
Since I took up bass, I hardly do any singing unless I put down the bass. When playing bass the best I can do is usually simple harmony. There a few reasons for this:
What you're playing on bass - at least bass in its traditional role
as a rhythm instrument and fundamental harmonic foundation - is
usually very far removed from the melody that you have to sing, both
rhythmically and harmonically: You're playing back-beats and
syncopation in opposition to the forward flow of the music led by the
vocal melody. You're focusing on rhythm and not melody - you're
listening mostly to the drummer and blocking out everything else -
your role as a bass player is entirely different than when you're
playing guitar. The flow of the chords on most guitar work (at least
in more modern music) is closely couple to the melody and works with
it - you're playing something that works harmonically and
rhythmically in sync with the melody line and vocals - vocal enhances
guitar and vice versa so singing comes rather naturally when playing chords on guitar. Not so with bass.
When you sing and play bass you are essentially playing two
different instruments simultaneously with two different roles that are almost opposition to one another: Your voice
and your bass. Granted, pianists and drummers do similar things all
the time, but I think there is a difference, because bass tends to be
a lot more physically demanding than the left hand of the piano or
the ride cymbal. Bass also demands a great of focus and precision because of of the central role it plays in the rhythm section - it is an unforgiving instrument playing a central role. It also carries its load on its own - bass has a
unique and solitary place in the ensemble, except for some occasional
help from the bass drum or an old school second guitarist (virtually
extinct today...). Bass is also loud and enveloping, putting your
focus on it.
- To add to the difficultly, you usually have to play bass almost
constantly. Many lead guitarists also sing lead vocals, but
they don't do much very challenging guitar work when they're singing, and
they have that luxury: They sing, then they play some guitar, then
they sing some more - meanwhile the rest of the band - the bass, the
drums and the keys, etc - keep things going while they sing. (Some extraordinarily skilled/talented artists - SRV and Jimi Hendrix come to mind - were often able to do both.)
Certainly there are some great singing bass players - people like Paul McCartney, Jack Bruce, Geddy Lee, Mark King, Sting and Lemmy come to mind. But all of those artists are on a completely different level in general in terms of their musical talent. I suppose if I had the talent of Sting I'd also be able to play and sing like he does - also write songs like his...
Interestingly, many in that list play bass in a non-traditional way. For example, Mark King and Lemmy use bass more like a guitar, playing chords to accompany their vocals.
Chris Squire (Yes) and Robert DeLeo (STP) - both great players - also did quite a bit of singing, but mostly harmony parts.
If you take a survey among bass players, you'll discover that most of those players who they considered the great, iconic players and stylists never sang a note, or at most just bit of harmony - people like James Jamerson, Jack Casady, Anthony Jackson, Chuck Rainey, Jaco Pastorious (he could sing very well and play, but he usually didn't), Bruce Babbit, John Entwhistle, Pino Palladino... (I'll leave traditional jazz out of it) . It's because they were dedicated to the bass, which precluded vocals - musically, or resource-wise - time and effort. They considered themselves bassists, not vocalists: A separate vocation playing an instrument with a special role of its own, not an accessory or subsidiary to something else. I don't believe you'll find nearly as many guitarists or keyboard players in any of the popular genres that assume such a role for themselves. (Drummers tend to be like bassists in that respect - the rhythm section is just different.)
On TalkBass.com - a very popular site for professional bass players, the consensus is that if you want to sing lead and play bass, your bass playing has to go on 'auto-pilot' - you know your part cold and can play it reflexively - without thinking about it all. Then you can focus on your vocals.
So, if you're ready to take that on, go for a lot of up front singing. Otherwise, probably better to stick to harmony parts.
Try it - maybe you've got the talent and you won't find it difficult. Start by playing bare bones bass parts to simple songs while singing the simple melody. Maybe try a simple melody of your own with the bass and metronome - humming even, so you don't have to worry about getting the lyrics right. Try playing a simple 12 bar blues at a slow tempo and just play roots and 5ths while you work out your singing. You must get to the point where you can forgot about what your playing on the bass entirely and not mess up.
Personally I never went that route, because I really enjoyed focusing and working on bass, although in terms of my usefulness in bands that held me back. If you want to sing and play bass and you are able to do it well, you will be a valuable, in-demand musician.
Although your question is about vx, as we've been discussing in the comments with @Basstickler, when you move from guitar to bass, you're not just playing a guitar with thicker strings in a lower register. You are a playing a different instrument with a different role: You're now playing a Bass. It's a Bass Guitar, but in most situations ( to exclude players like Stanley Clarke, Victor Wooten and Mark King... ) , what that means is essentially an upright bass in guitar form, which is how the instrument was originally conceived: As a portable, easier to manage, louder sounding version of the upright bass.
Its musical role is the bass's role: A rhythm instrument that keeps time, provides rhythmic interplay and states the underlying harmonic fundamentals of the music. It's an entirely different role than the guitar, a lead-solo instrument and/or one that's playing chords that enhance the melody line. To be a good bass player you need to aware of that difference and adjust - you're not playing guitar any more. That difference in the role of bass is the biggest factor making singing more challenging for most than guitar and vx.