Not sure if things had change.
SPF will inform you the demand vocations during mass briefing on your 1st meeting (verify certification, written test & etc). If you chose the demand vocations (during my time is K9 & PCG), you will end up there or else most of the time, you will be place under NPCO vocation.
With a Poly Dip or GCE A, you may end up becoming a NSPI for your NSF but if you sign on, you are only a SGT(2). You may also start from SC and final rank maybe only CPL which is a waste of time (2 years of NSF) before you intend to sign on compare to sign on direct as SGT(2).
From what I heard from my ex-REG friend (NSF then become REG before left force), if you decided to sign on just before you ORD, you may have those type of contact that you can quit anytime (Eg: 1 year) without any liquidated damages.