Fred says it is ok say hello and hand over a business card, but make it quick and don’t pitch. That makes sense to me. All decent VCs love meeting people and are in the business of getting to know companies will be happy sparing a few seconds to share contact details.
People spend a lot of time worrying about how to behave around VCs, about how or when to make an initial approach and then at subsequent chance encounters about how much to pitch. The right answer depends on context – are they with family or is it a work social, how many other people have pitched them that evening, do they look tired, are they standing on their own or talking to other people etc. VCs are humans too, just like you, and the best way to figure it out is to put yourself in their shoes and think about what you would like. If you do to others as you would have done unto yourself you won’t go to far wrong in this world.