- I've seen migrants (most likely void ones, but I haven't paid enough attention) of both genders without a spouse but with a child, so it does happen.
- As far as I understand, there is no pop limit for vampires to show up (I thought it was 50, but got one before that, asked, and was informed there's no limit). However, vampires are cursed during world gen or within the ongoing world, so logically, they should not be able to be void dwarves, but that's just my conclusion, not based on any actual knowledge. I believe I've heard vampires' official identity can be married, but the spouse is never present within the migration wave, nor is any relative present within the fortress (which means a kid or a spouse with them would rule a vampire out). This is second hand info, so take it with a grain of salt.
Thus, taking Sver's info into consideration:
The couple should be the ones who are definitely NOT vampires. It might be that the two first waves never contain vampires, but that's purely a guess.
A "spouse" of a vampire would have to be under the mental influence of the vampire to claim to be married to the vampire, which is a power DF vampires do not have. The other logical alternative would be that they're both vampires who have decided to hunt together, something I very strongly suspect is not supported by DF.
Screening all your migrants for tells or counter tells is a good practice.