From the description
Right now you're not producing anymore drillers, because you have more than your generators allow you to create. If you could find a way to start producing drillers again, Vintgar will have enough defence against the incoming sub.
From the hint
Some specialists increase your electrical output.
From me
Vintgar has 10 shields already, and the enemy sub has only 11 drillers. So only 1 driller on Vintgar would be enough, you've got plenty of time to get a specialist you want! There's no problem if the first batch of specialists can't help you.
(Actually, once you find the right specialist, you will still be able to produce 18 drillers before the sub arrives, so you'll defend Vintgar by a large margin.)