I guess this would go here.

I am wondering if I have to have beat the Vanilla MQ, or if I can play Dragonborn having just informed the Jarl of Whiterun about the dragons.....

I think you need to have to have completed the horn of .----- windcaller for the greybeards. But that is just what I have read on here.

Would I still be able to go to Solthseim (Spell check?) and is that just to do Dragonbor's main quest, or is it required to even go to Solthseim?

No.
You do not need to start Skyrim MQ in order to go to Solstheim.
You do, however, need to release the dragons in order to start the Dragonborn MQ.

- The character I've taken to Solstheim has not started the MQ.
- Both Skyrim and Solstheim are currently dragon-free zones.
- To get to Solstheim, simply head for Windhelm's docks and book passage. Easypeesy.
- Also; there is plenty to do on Solstheim without ever touching the Dragonborn MQ.
- I'm having a wonderful time without having done anything on either MQ.

Hope that helps

Exactly the answer I was looking for, and hoping for, thank you.

Glad to help.
There is quite a good deal of content for a non-dovahkiin character to enjoy.