If you sign on anytime during your NSF time, they will backpay
you a certain amount and consider the whole period of NSF time as
having served as a regular (since it is already full-time).
Vocation - cannot choose. You can request your preference, but
SAF has no obligation to keep you at a certain vocation or give you
what you ask. They will freely deploy/vocate you to their
needs.
Request to your direct superior to talk to your unit's S1
(manpower officer) or DyS1 (deputy manpower officer). They will
give you the advice and details.