I am writing a book with vampires and there are a lot of flashbacks. How do I make them different from the normal text. Italicise them? Italicise the dialogue only?

If there are a lot of callbacks to the past in your text, it may be better to find ways to incorporate them into your writing rather than to have strict flashbacks where you break the narrative and actually flash back to a different time. For example, you could have something trigger a memory for a character and then have the narrator describe the past event within the actual narrative - no italics or anything needed. Let me show you what I mean.

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The wind blew cold and strong over the city streets and Irina pulled her jacket closer to her. It was a silly gesture, she knew. The cold meant nothing to her now. It had meant nothing to her in some time. But still, it brought some comfort to her, and Yuliana’s scent still clung to the lining, like roses in bloom, transporting her back to the garden where they had first met. 

It had been warm then, and Irina had been able to feel that warmth as she lingered in the garden, watching the maiden who tended to the flowers. “What are those?” she had asked, moving closer as the younger woman looked up, eyes widening slightly upon seeing the lady of the house in her presence. 

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Okay. Not going to go much further than that, but you can see my point. The present - Irina in the present, in the city - moves into the past. And then you can have the flashback scene take place. The, you can either end with that flashback or simply acknowledge moving back into the present. So, one final example - 

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As she walked, the memories faded away. Yuliana remained unchanged, thought Irina as she continued through the streets. She was still that same young maiden she had found in the garden, so long ago. This curse was not meant for creatures like her.