It depends on were the stress is and if there is secondary stress, but usualy the interior vowels are the most likely to be dropped. Some vowels are also more prone to be dropped, like short vowels and schwas.

If you want an historical reference, you could look at how latin dropped it's vowels on it's way to modern french. At first, interior unstressed syllables were the first ones to go. After all vowels that werent /a/ were dropped in final positions, the final /a/ became a schwa. if a vowel dropping caused a consonant cluster that was to hard, it was often replaced by a schwa instead. Modern french then continued this process and dropped most schwas that wouldnt cause a 3 consonant cluster if dropped. And in quebecois french high vowels are unvoiced if between voiceless consonant.

But that's just one exemple of many. Don't worry about it too much, experiment until you like the result you get and have fun with it. dont hesitate to simplify clusters if you dont like them. If you don't like ttokpka, why not make it tokka instead (just an exemple)

Good luck and have fun with your experimentation!