if you want to overvet, you need have to plan first,

1) buy some expert packs or collect in drops
2) burn all experts until its all gone (there will be a chance that you'll get less vets or more vets)
3) replenish your experts using the packs you bought/recieved

Do not, if your just shy of 900 SD then just play a little more and level up to reach that 900 SD

Only Burn experts if you can reach 200 vets . 1 vet = 4 experts, so it will be the same as having 100 vets and 400 experts. Then you wont see any lack of performance.

I also did what you are thinking of, i only got a few extra vets and it gimped my crew, i went from 900 SD to like 400 something and i no longer reached the speed cap which made it harder to play. So now i just bought more experts and got my IJN crew all 200-250 vets, i burnt all the experts on there so i just have vets and rookies. With AA ill make up those experts later, but ill still perform as if i had a standard B100VE crew and it will only get better as the experts increase.

Personally I get them to 100 vets and leave 'em that way until BB6. At lower levels the experts will do more for your crew then trying to overvet all the time. Running BB1-4 is difficult enough with at level crew, much less gimped crews. I would only recommend vetting past 100 if you have the patience to get in an AA boat for a few days afterwards or have the expert packs to replace them. Otherwise wait till BB5-6 to overvet. You won't be getting an improved boat soon, so only your crew can improve it at that point. You're also competitve at that level, so the ship can somewhat make up for the gimped crew instead of at lower levels being highly dependant on any ounce of ability you can milk out of them. 

I would wait until there is an event with vet conversion bonus.  Sometimes they will offer a 200% or 300% converson rate.  I wait until the 300% for a better return on burning experts.

not quite on the point ..but you say you have sailors boosted to 100 vets each?

You may know this but you should get to 109 vets before having to burn for more vets

99 vets then  one 10 boost =109

 

i'd suggest waiting until your SD / speed isnt crippled by the actions, and do so for 1 or maybe 2 (or a rep & a engy) at a time, not all of them.

if you got a good crew, which it sounds like you do, you should be able to do pretty good allready. imo overvetting isn't really that hard needed & only something you should/can do if you have money to spend or a lot of time to spend on AA-ing.