Most of what you wrote is theoretically playable, but consider that it's extremely risky, because it requires the player to:
- use the same 4-mallet set for both instruments, which may not be advisable both for sound requirements and instrument sensitivity;
- use the same grip technique for both instruments (more on this later);
- have both instruments very close and positioned compatibly to their movements, which is not always possible: a 90° position between them allows fast changes, but the stage may not allow it;
- have a copy of the part for each music stand (this really shouldn't be a problem, but you never know): obviously, if the player is skilled and responsible, they may learn the piece by heart;
In reality, I would seriously discourage that writing.
Analysing your part, I'd say that the only "safe" change is between bars 11 and 14. Everything else is strongly discouraged and a good performance would require two players instead.
Remember that while vibraphone and marimba are similar in concept, they are obviously different.
The player may prefer different techniques on each instruments: for instance, the Burton grip on the vibes, and the Stevens grip on the marimba.
While both techniques may be used on the other instruments, the player may not be so used to that usage, mostly because mallets of those instruments have different lengths, weight and balance, and your piece is also relatively fast and difficult, leaving less margin for adaptability.
Using the same mallet set may also be a problem. Besides the possible damages that could affect the instruments or the mallets, the required sound may not be compatible.
Switching instruments also requires some time, both physically and mentally. Even assuming that the player keeps the same mallet set, you need to consider:
- the time to actually go from one instrument to the other;
- the reasonable time required for the vibraphone sustain pedal, in order to avoid abrupt stops;
- the mental time required to the player to "adjust" themselves to the different keyboard layout;
- the sound stage floor (don't underestimate it, a player running to another instrument can make some very loud and annoying sounds);
Finally, remember that, as composers (and arrangers/copyists), we should always try our best to keep the musicians at their ease, leaving them the necessary time and conditions to be able to play as best as they can.
Continuously switching between instruments in a matter of seconds certainly does not meet that requirement.
Don't be afraid to write more parts for separated instruments (but don't write too many). A good percussionist, possibly the principal, will eventually be able to merge what required based on their possibilities.
A possible alternative is to write parts with multiple staves for each instruments: in this way you can still have "one part", and the percussion players will eventually manage that on their own.
In any case, consider the following rules of thumb:
- leave at least 4-5 seconds between percussion instrument changes; possibly, double that;
- double that again for changes between "incompatible" instruments (for instance, keyboards and drums) or even more for instruments that have incompatible accessories (eg. mallets against pair of cymbals);
- test it yourself: if you're not a percussionist, you can safely consider your reference as the lowest assumption for that change, so if you're not able to do it, the player shouldn't have to do it (even if they could);
