Yeah it’s a pain.
I had to take a small pot of pre-dissolved cacao in my rucksack along with the berry RTD when I went on a hike. Not ideal. But it worked.
The vanilla RTD was much harder to flavour. Like you i thought it was really disgusting.
First I added 3tsps cacao. Still bad.
Then I added peppermint oil. Better.
Then I added a tsp of the flavour boost (which is what your asking about)
I then necked it in one and it was ok.
I can’t really say how well it dissolved because of everything else I’d added and how fast I drunk it not wanting to waste.
The flavour boost did seem to mix in ok.
But other people have commented that adding powder boosts didn’t work well as it didn’t dissolve. If you are able to mix the flavour boost into a little bit of water first then add it, it would be fine.
Flavour drops are an option.
I have some in my cupboard: mint, hazelnut, cherry etc
Only trouble is, that’s adding even more sweetness.
Instant coffee and or cacao are the only things that will reduce the sweetness. If you don’t mind the sweetness and just need flavour, try flavour drops.
Coffee would actually probably work really well in vanilla RTD and is easily available anywhere you go.
I only had one bottle of vanilla to try (and won’t be getting more!) so I can’t test it for you.
Worth trying tho I reckon