I've never liked using a mightyvac, they induce more issues than the supposed simplicity they are supposed to have. It will also tend to pull any tiny port blocking stuff in the m/c into the valve, making things difficult. There is clearly air caught between the left and right caliper, somewhere near the junction at the M/C because you are using a 2 line system.
Also trying to pull fluid down thru one side, with the vac, allows air in the opposite line to be pulled back and forth.
The vac will also pull air in thru the threads of the bleeder fitting when the system is "closed".
try this.
go to one side and begin pulling fluid at one caliper. Have a clean bottle there with a substantial amount of fresh fluid. when the hose is full, raise it above the bleeder with the bleeder locked down, now quickly place that full hose into the jar of fluid without making an airbubble in the line, or as minimal of one as possible. with that hose now submersed crack that bleeder open again, go to the other side, and vac that side, drawing fluid from the bottle as the supply, don't mess with the m/c, just keep pulling fluid up and over. this should purge the lines between both calipers. Don't run the bottle dry...close that bleeder before getting distracted.
If you have no resistance at the lever, and it goes totally flat, doing the bungie thing won't help at all, that only works when there is at least half a lever resistance, and it is spongy.
after you get those lines purged side to side, go back and do the normal pump/lock/repeat bleed method.
in essence what you are needing to do, is to pull fluid from one set of calipers, along with the air entrapped there, up to the junction (bubbles flow up to top) then back down and out of the opposite caliper. Trying to push air out of 2 lines into 2 calipers is like chasing bubbles....