As stated, Summon Undead is an evil descriptor conjuration spell. However, that is irrelevant anyway. The feat itself states "At 4th level, the pale master can use create undead as a spell-like ability once per day.". Not only that, but the Summon Greater Undead feat states "At 9th level, the pale master can use create greater undead as a spell-like ability once per day.
Playing the devils advocate here.
The rule is 'RP your character sheet' thus if my character sheet states here that summoning an undead is not an evil spell it simply isn't.
Regardless of the fact that it is an evil spell by the source book
You are incorrect about he level 9 palemaster ability.
I have it right in front of me.
(The palemaster may summon an undead like creature once/twice a day as a spell like ability)
On this server it says nothing about creating undead or the spell working as create undead.
It clearly states you are summoning an undead.
Keep in mind the Palemaster we have here on BGC is not the same Palemaster of Libris Mortis.
Libris Mortis palemasters don't even have summon undead + summoning undead are evil spells.
For the purposes of mechanics, yes, your vampire takes a summon slot instead of an animal companion slot. However, you are none-the-less summoning the same undead creature (unlike the normal create greater undead, which summons a random choice between a vampire warrior and a vampire warlock).
BGC is very clear about when something is a cohort or not.
Thaumaturge his cohort will remain after resting, can be present next to a summon.
Just like animal companions.
When it doesn't state that it is a cohort than it simply isn't.
BGC has been strict about it's rules, examples are tieflings rping tails or people rping a half something.
Just like that in the end it is a medium RP server and spell restrictions aren't enforced neither are armor restrictions.
I do think it is a shame, just like nwn2 is very limited when it comes planar allies/familiars and in general stuff that adds extra flavor.
Every neutral wizard has a fire mephit ally/sylph etc , every PM an elite vampire female etc.
It's boring and reduces the RP value, but that's how it is.
Also by following logic vampires shouldn't be out during day light, you must be an incredibly stupid palemaster to pick out of all undeads a vampire to summon outside in the hot bright sun.
In the end it's better just to roll with it

.
So we can state that on this server with these rules, you can summon an elite vampire as palemaster, with out having your alignment shift towards evil.
Regardless of the fact PM's can't summon those and that spell on it's own is evil ((+ it needs components).
Anyway IC wise you would probably be called an evil necromaner, and get chased by the holy mob with torches and pitch forks.