Alternatively, I'd be fine with "A drow vampire with 8 or more hit dice can also call a single drider."
Come to think of it, the Create Spawn in the current writeup only produces standard vampires and vampire spawn - it doesn't create drow vampires!
We could kill two spiders with one stalagmite by adding a line that it creates drow vampires if the victim is a drow with 8 or more hit dice. That way drow vampires are always created with a higher CR than a drider. It'll also give them a minimum HD of 8, which is the same HD as a standard AD&D drow vampire (or a regular AD&D vampire, for that matter).
e.g.:
Children of the Spider Queen (Su): Drow vampires command the monstrous arachnids of the world and once per day can call forth
1 drider, 1d6+1 spider swarms, 1d4+1 Large Monstrous Spiders, 1d3 Large Monstrous Scorpions, or a pack of 3d6 Medium Monstrous Spiders as a standard action. (If the base creature is not terrestrial, this power might summon other creatures of similar power.) These creatures arrive in 2d6 rounds and serve the vampire for up to 1 hour.
Create Spawn (Su): A drow slain by a drow vampire’s Con drain rises as a vampire spawn (see the Vampire Spawn entry) 1d4 days after burial.
If the vampire instead drains the victim’s Constitution to 0 or lower, the victim returns as a spawn if it had 4 or less HD and as a vampire if it had 5 or more HD.
If the victim is a drow elf with 8 or more HD they return as a drow vampire.
In all cases, the new vampire or spawn is under the command of the vampire that created it and remains enslaved until its master’s destruction. At any given time a vampire may have enslaved spawn totaling no more than twice its own Hit Dice; any spawn it creates that would exceed this limit are created as free-willed vampires or vampire spawn. A vampire that is enslaved may create and enslave spawn of its own, so a master vampire can control a number of lesser vampires in this fashion. A vampire may voluntarily free an enslaved spawn in order to enslave a new spawn, but once freed, a vampire or vampire spawn cannot be enslaved again.