I hear rumours that it's impossible to obtain one after you've finished the Vampire MQL, however i stumble across Dwarven Bolts every time i loot a Dwarven Sphere... This makes me curious and intrigued as to why i'd be able to get the bolts but not the Crossbow itself, which is sad.

You can't.

Yet you can become a VL even if you finish with the Dawnguard.

The DLC was a tad bit biased, methinks.

Can you get the amulets of night power, the ring of the beast, the ring of erudite or the powers of the blood chalice by finishing the Dawnguard? No you can't. If your not an archer joining the Dawnguard doesn't really offer much. The spells aren't that good.(and with pacify you can get them as a vampire.) So that just leaves huskies and armored trolls. The Ring of the Beast and Erudite work in all forms. So even if you don't use the VL form you can benefit from those rings.