This is a re-occurring question. Here is the best chance for success:

All factors must be met:

Win the Battle
Crewmen levels up
You dont sink
Your ships was effective in the battle, IE you done some damage, you shot down
planes, you sunk a ship.. something...

Thanks for the tip. Going to give it a try

Oh wow. Thanks for the hint. I always thought vetting them later on would be better. Just
tried with my low level US crew (all sailors between 40 and 47) and it's working very
nicely. Success rate was about 50% which I think is pretty darn good, because vets should
be more than twice as good as experts are.

Thanks again, and recced of course. :)

i have found that if you win the battle that you crew lvl's u have a way better chance
of getting 10 vets.. stop when u get ur vets ... also rember the 40% rule. you can
not have more then 40% of crew a vets...

actually I would think anouther variable is on if you can afford to buy vet and exp
packs too.

uhh...u can vet your crew at any level?

the only difference i found is that the more your sailors are classed, its harder to get them vets.