Here are a couple of options and some info...first the info. The rate of seeing a vampire at a downlot is low (thinking its 10% of the time) if you stay about 1 hour (game time not real life) and dont see one you can move to another lot and try there. Also, you can make a sim who is attracted to vamps and then pay gypsy 5,000 (make sure its after 7 pm game time) and you have a chance of her spawning a vamp as a date

Once you find one make friends with them (easiest way ive found is to eventually have them move in and then you can control them...also can use your influence to get them to bite someone)
Methods to get vamp: use the control + shift + c keys and then type in boolprop testingcheatsenabled true. then left click on sim and from menu select make vampire or you can click on mail box and spawn one (I think this still works).
Other methods: (my personal favorite-still has animation) Vamprotoxin-A 'Essence of Vampire' found at
http://www.modthesims2.com/showthread.php?t=125591&highlight=vampor you can use the following: Artifact Of Cain found
http://www.modthesims2.com/showthread.php?t=95121&highlight=vampor Make your Sim a Vampire Object found
http://www.modthesims2.com/showthread.php?t=175240&highlight=vampSome other good mods are the sunblock found
http://www.modthesims2.com/showthread.php?t=162163 and Carrigon's Vamps walk in day mod (sorry cant find link right now and it needs to be updated).
Hope all that helps.
Duncan