Hi- I love this topic because I am so protective of my daughter, I tend to worry about this a lot!
I think your daughter is the perfect age to set boundaries and expectations. Avoid having a long, drawn out conversation with your daughter about this, but in lots of little conversations, let her know your expectations/values. For example (add your own expectations here), there is no kissing of any kind. If a boy likes you (and you like him), he can smile at you, draw you a picture, play a game at recess, etc.
It's important to talk a lot about what's not ok with you, and why. It's equally important to let her know what she CAN do. If she's a little boy crazy, you can't really stop that. If the boys like her, ditto. Your job is to give her the parameters to operate in so she doesn't flounder.
I hate to judge the little boy, because I don't know the whole story. Maybe a little weird for him to be interested, but if they were at VBS together and not really grouped by age, and she looks older, it's probably innocent. I do always say if you have a weird feeling, trust your gut. If you do decide it's innocent, great. I would not send her on a play date at his house, though. If they really want to play, I would do something like meet at the park. At the very least, she should be supervised at all times.
FYI, luckily schools can back up some of the affection rules. I know at my daughter's school they don't allow the kids to hug or kiss at all, made it easier to have some back up!
Good luck!