It is common to play double-stops, certainly in solo pieces; and indeed also quite common to play 3- or 4-voice chords (these need to be "broken", i.e. you quickly excite the low strings and let them ring, then pull the bow up and sustain the high strings).
Indeed the multi-stringedness might have something to do with what you call "synthetic" sound you're suffering from: most of the time, even if only one string is actively played, the other are free to vibrate and cause sympathetic resonance. But this influences the sound only subtly anyway, so it probably isn't your main problem.
This part of the answer doesn't apply to your question, I thought you were talking about an actual cello when I wrote it, not a VST one.
"straight bow" is more to the point. This is orthogonal to how many strings you're playing... literally: a straight bow is when the angle between the bow and string is 90°. This is normally the best angle to get a good solid sound1.
Angle is just one of many bow parameters:
- Default position for the bow on the string is between the bridge and the beginning of the fingerboard, a bit closer to the fingerboard but by no means above it2.
- The bow itself can be angled axially, i.e. you can either have all bow hairs flat on the string or you use mainly the upmost hairs. Again, straight is a good thing to practise first, though it's generally ok to have an angle here (it gives a bit of a sweeter sound).
- Pressure is of great relevance. You'll often hear the advice "use your arm's weight", i.e. you should be relaxed yet excert a firm and steady pressure on the bow against the string.
- Same goes for speed. You can move the bow fast or slow, but you need to be deliberate about it, and consistent.
Controlling all these parameters in realtime is, obviously, not easy. Apart from the inevitable practising effort, you should make sure you're holding the bow the correct way. You'll find many pictures on the internet, but really – asking a teacher to show it to you is much more effective. There are many bad habits you can develop in the right hand.
So that was only the bowing aspect. Of course, when playing proper cello pieces you also need to be very accurate about the left hand fingering. But before you blame it on the left hand ("more vibrato" is definitely not the solution to your problem, though it might initially seem to help), make sure you get the bowing right.
Now, the above block hints at what's the real problem with VST strings: there's a darn lot of parameters a good string player controls simultaneously, to get the right sound at every moment in a performance. If a note is repeated, even if it's nominally the same length and dynamic level, it'll sound different because the bow is moving the other direction. If a passage is played quickly / staccato, the player will put in more pressure on the beginning of the tones so the notes develop fast enough, whereas in slow / legato it's more of a soft fade-in. For some intervals you might need to perform a particular position change that gives a barely noticable, but characteristic sound (in particular on the cello, position changes can make quite an audible portamento even when the player hides it).
Today's top-notch plugins, e.g. the Embertone Blakus Cello, will generally offer ways to emulate all such effects. But it'll only sound natural if you actually know how to correctly place the effects.
There's one particular thing I consider paramountly important but is largely neglected by vendors of such plugins: intonation. Though players of guitar or piano will regularly claim otherwise, string instruments do not play in 12-edo tuning. Every single note is actually fine-tuned to some pitch that can depend on lots of factors; it's normally said that string players switch between Pythagorean tuning and just (5-limit) intonation, depending on the context.
1Good players will often deliberately angle the bow in various ways, but (unless you want to provoke e.g. a particular screech sound) this must be combined with controlled moving of the bow point. Before you try any of this, make sure you know how to bow straight.
2You'll sometimes find an indication "sul tasto", this means you should bow right at the edge of the fingerboard. Again, this is something to use as an effect.