You can “conflict bid” your vacation or your training.
Bid a line that either totally or partially overlaps your vacation.
Then you can do what they call a “vacation slide” which allows you to move your awarded vacation (week) so it covers a more beneficial part of your awarded line.

Its hard to explain without drawing it:
Lets say you get awarded a line which starts the 1st and ends the 17th.
So 1-17, your awarded vacation is 13-19, you can now slide it towards the beginning of the month, let’s say 10-17 or 7-14.
In the latter case you’ll get the last 3 days (14-17) awarded as they can’t use you by for anything as you need to travel, preduty rest and back before your day off starts.
So yes, if you have the base seniority you can’t finagle three weeks with one week vacation.