Lizard said:
How long it takes a vampire to make spawn can impact combat. If it's a minor action and he has minions to spare...or, as others have noted, if the vampire is alert to invaders. If it takes a minute, he can turn his domestic staff into spawn while the heroes are distracted with a single fight. If it takes an hour, he might have time to get one new ally before they get to his sanctum. If it takes a week...he's SOL..
Well, he doesn't have the ability to do it it combat, for one reason or another. Otherwise it would be listed in his stat block.
The rest of those things are meta-game concerns. And very simulationist ones at that.
From a gamist/narrativist point of view(which is pretty much the default assumed by 4e, as far as I can tell) the answer to those questions are simple:
Does an extra vampire make the encounter that the PCs are going to fight too high a level to be balanced?
Do I want the NPC to live for my plot later one?
If the answer to either of those questions is yes, then the vampire can't create a new ally(or doesn't think of it or fails or is distracted or...whatever).
When I DM I normally have all of those things planned out way in advance. My DM notes for a situation like that would likely read:
"The PCs will likely have found enough clues by this point to realize that the hostages were taken to the old warehouse on the west side of town. If they haven't, an old man comes up to them and tells them he saw one of the hostages through a window in the warehouse. The Vampire Lord has one of his minions following the PCs and when the minion sees that they are heading in the direction of the warehouse he heads back and warns the Vampire Lord. He knows the village much better than the PCs, so he'll get there a while before they do.
The Lord will then kill all of his hostages in preparation for the combat against the PCs. He'll turn them all into spawn. This means the combat will be against the Lord and 3 Spawn. The Lord is Elite so that will make it a perfect 5 monster encounter."