i dont know of every method possible to crack and older version of
vector but you'd be looking at security issues such as glibc, openssl,
bash, everything from mozilla, the entire LAMP/LEMP stacks, samba,
xorg, the lsit keeps going. so no its not necassarily more secure than xp
even though you most likely wouldnt run LAMP/LEMP on it youd still
have apps that need mysql/mariadb and php.
you could probably lock it all up tight with iptables or a hardware firewall
and of course not running any services helps too, cant exploit httpd
if its not running for example. youd still have a huge gaping hole
just from running many years old firefox. and you wont be able to do
much if any updates.
ive got a p4 that runs vector linux 7.1 just fine and i just use Nouveau.
with all that being said you certainly can run an older vector and the nvidia driver
but in my personal opinion the bad outwieghs the good.
if and when you do find an older release of vector to run google the version of xorg
for security issues and make an educated decision on what youd like to do.
im not trying to convince you one way or the other im just bringing up some issues
that im aware of so that you can make the best choice for yourself and what youd like to do