You don't have to worry too much about vocations - since it is
not within anyone's control.
The SAF posts and deploys manpower according to their needs.
Ranks and appointments are according to manpower needs as well
(though fitness/qualifications are a criteria, but does not mean
all who fit will be officers/specialists).
For example, to qualify to be an officer an 'A' levels or
Diploma is required. Most Singaporeans have these, but it won't
make sense to make everyone an officer, then they will have nobody
to lead. Therefore where one is posted to is largely a matter of
manpower needs.
Security clearances are complex matters and cannot be explained
over an online post. There are many factors to it.
There're no restrictions on PR being officers/specialists. I
personally know of two officers who are PRs.
Since PRs are not citizens and always have the choice to give up
to return to their country, or even joining another army, the SAF
obviously have reservations posting them to sensitive units or to
hold key staff appointments (such as Commander/Commanding Officers,
Intelligence Officers, etc). These are unspoken, understandable and
obvious.