Please read this post, and the ones following it, for the details of what's happening here:
Rules for Distance between Borders calculations - DipVassalisation and RM
It's because they're on a different continent, and therefore your adjacent cored land is not contiguous with your capital and doesn't count for RM distances. This only applies to the RM Distance Between Borders modifier, not the DipVassalisation modifier.
Correct regarding the no-stab-hit for breaking RM as Curia Controller.
Note that you could have checked the Base Tax requirements for DipVassalisation without taking the RM - just hover your mouse over the green tick/red cross on the Offer Vassalisation button to see the modifiers that are applying. You can do this even if the button is greyed out, so you can check before you get a RM and indeed can check the modifiers that apply for any nation even if you could never vassalise them (e.g. due to not being able to get an RM at all.) And this enables you to check requirements without having a diplomat available, or when you can't send the Diplomat for 30 days, etc.
Same applies for all Diplomatic buttons. The only action where you can't the modifiers at all times is for a Peace Treaty / Sue for Peace.