I'm really looking forward to Dawnguard but my problem is, well can I be a Vampire Lord and still get / use a Crossbow?

Probably yes, but I think that you will not get the upgraded dwarven crossbow. Not sure though

It is assumed that you can, considering them being craftable and everything. (And considering that it seems like you start the questline joining dawnguard and later turn to the vampire side if you wish).

You'll find out for sure tomorrow...