A few things for contemplation:
It's worth examining what exactly it is you are skeptical about, and why, especially as regards Vajrayana and rebirth, do you know either one well enough to decide definitively whether or not you believe in it, and why is it important to you to define that? With the things you are skeptical about, you need to figure out what they are actually saying first, then you can decide. If you are just having an immediate reaction to the idea of rebirth or to Vajrayana's colorful practices, that is not really skepticism, but might be also tied up in some cultural imprints.
Of course you can practice it in whatever way you want, like most things it's unlikely anyone is going to ask you to prove you believe in rebirth before attending. It seems some Tibetan centers have days that are mostly Shamatha/Vipasanna sitting sessions as well.
Not trying to be facetious, I've been in your shoes and it is the right place to start from, just saying, be as willing to question your own ideas about the importance of your beliefs, and your own cultural baggage as you are willing to question rebirth and Vajrayana practices. I truly believe (not saying it's you, just saying i've seen it) that many people actually have a reaction to Vajrayana partially because it is so culturally foreign, and then they incorrectly label that reaction itself as skepticism. Once you actually start learning the philosophy behind it and say "no this cannot be so" that's actual skepticism, it involves some level of analysis an rejection, not just gut reaction - which truthfully seems to be coming from the more puritanical side of our cultural baggage than anything else.
If you are both simultaneously drawn to the practice but feeling this tension, there might be more at work than just your rational mind, and it's worth including analysis of your own beliefs in there alongside your questioning of Vajrayana, rebirth, and whatever else. Again i'm not trying to sway belief one way or another, just saying that accepting one's pre-existing cultural beliefs is not the same thing as honest inquiry, and if you haven't already been through the honest inquiry..why not start with that?
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