I know I am really late to this conversation, but my group recently started using Mythic Adventures.
They way I understand this is Vital Strike would run like this:
Fighter with a Falchion and a +4 STR mod (not adding other bonuses for simplicity), also using straight book rules for damage multipliers wherein you roll the damage again for x2
2D4+4 + an additional 2D4 for the Vital Strike damage.
Whereas Mythic Vital Strike for the same stats would work like this:
2D4+4 then another 2D4+4 for the Vital Strike damage.
Now Improved and Greater with the Mythic would work like above but rolling a 3rd and 4th set of damage dice respectively.
I know my group doesn't typically roll the damage over and over, but that's the book way, and I want to have the book method down before house rules get thrown in.
Thanks