In my experience, I don't see these random vampire attacks until I get approached by Durak or overhear guard dialogue about the Dawnguard at level 10. So you could theoretically sit at level nine until you're ready to play Dawnguard, and you wouldn't have the vampire attacks.
May I ask why you don't want them to attack randomly, even if you're not going to do Dawnguard right away?
Vampires will start ambushing the holds at level 7. Since I have DLC I tend to limit my level to 6 until I've really gotten situated then level all at once when I feel confident enough to take on the vampire menace. If I'm lucky enough I get a visit from both Vampire and Cult priests and they tend to wipe each other out.