You witness an attack, then hear about the attacks, or you find that one orc fellow. If you show no interest, you still get attacks.
I take it you haven't made multiple new characters to actually test it out?
I have one character started just before Dawnguard was released and two other high level characters who have now completed Dawnguard, one with the Dawnguard, one with the vampires. So yes I am pretty sure you are not going to get any surprise attacks, you get a warning first that the vampires are attacking towns. Then you might get town attacks, or you might not. My highest level character went through the DLC first, an over achiever who had done everything and was used to being attacked by mass spawning enemies (came with mounted combat) and dragons any where outside of walled towns. He was the one attacked on the very first town he entered once Dawnguard was installed, the guard was talking about attacks at the gate, next thing there were vampires and vampire thralls all over the place. Like everything else he got a high level of attacks, so decided to join the vampires in the hopes of getting a quieter life, it worked, although he still got a fair few town attacks and is still fighting off Dawnguard, joining the vampires did get him a lower rate of vampire attacks.
Aussie went through second, little adventurer who hated being told what quests to do, she had all the houses, was started so I could get the Oblivion Walker achievement which had been done and went and did the Stormcloak campaign. After my first character Braggen's problems with dragons, I was not interested in having another character being chased all over Skyrim by dragons. Aussie never went very far with any other questlines and never let Alduin go waking no dragons. Aussie got vampire attacks in the wild, heard about the vampire attacks from guards, told the recruiter no thanks, not interested. She went up quite a few levels doing smaller quests and collecting stuff, she had no vampire attacks on towns, yet a high rate of attacks in the wild. Eventually she went off and joined the Dawnguard, she had attacks nearly every time she was in Windhelm, although usually not when she first entered. She also got attacks in a lot of the smaller towns, yet could not get an attack in any other walled town but Windhelm.
My third character started Dawnguard around level 10, and only heard about the attacks at level 18 when that recruiter eventually caught up to him. He has not had any attacks on towns yet, and none in the wild. He is doing the Stormcloak and main questline together and is not that enthusiastic about chasing vampires unless they give him a reason. I will wait and see if he gets any attacks, no attacks he will not be doing Dawnguard. I will go start a fourth character to try a low level starter. My third character is an Orc and he is hopeless at fighting, I am never ever playing an Orc in Skyrim again.