It says in the template itself that a character must be fifth level to become a vampire, lower than that and they just become vampire spawn.
Strictly speaking, of course, what it says is that
a character drained to 0 Constitution or less returns as either spawn or vampire dependent on hit dice.
But the template can be added to
any humanoid or monstrous humanoid; compare, say, the Death Knight template, which can be added to any evil humanoid
of 6th level or higher. The Death Knight has a restriction on the template itself; the vampire only has a hit dice restriction on vampires created by being drained to 0 Con.
So if there are other ways of becoming a vampire (campaign specific, presumably), for instance: being born a vampire a la Blade; drinking the blood of a vampire and subsequently dying a la Chelsea Quinn Yarbro - then perhaps the hit dice restriction does not apply.
But that's up to a DM. If the
only way to gain the vampire template is through Con drain, then yes, only base creatures with more than 5HD will be vampires. But it's not a restriction on the template itself.
-Hyp.