The only reason to lengthen your run is to vault higher. It's not going to improve your technique. In fact, it won't be quite as good.
If you don't compete again til next year, you have plenty of time to lengthen your run.
I'd focus more on your plant/takeoff/swing technique from 5 lefts. You'll learn these vault parts much faster with a shorter run ... even if only because you'll have more energy and can do more reps per practice.
But there's other reasons too ... mainly that everything is a bit slower in a shorter run. If you increase your run length, you speed up your plant/takeoff timing, which is more difficult to learn ... because everything's happening too fast.
So yes ... talk to your coach ... but have your goals clear for the summer before you go off trying to just improve your PR without any consideration for improving your technique.
My guess is that if you improve your TECHNIQUE this summer, then next Spring ... when you lengthen your run ... it will be so much easier to set a PR because your TECHNIQUE will be BETTER!
BTW, there's nothing stopping you from improving your PR this summer ... from 5 lefts!
Kirk