Dead circuit, couldn't find problem so tied into second circuit

My girlfriend lost power to several receptacles and lights in her home while using an old vacuum cleaner that pulled a lot of electricity. She told me that first the room lights and power to the receptacle she was using turned off, a minute or two later they came on, and then they shut off again for good and were off for the better part of a day until I got there.

I cycled all the breakers with no effect. I then removed the panel cover and checked each breaker with a circuit tester by using a tester on the circuit breaker screw and ground, and checked each hot wire while confirming I got a light on the tester.

So the breakers are all good, no broken black wires connected to the breaker screws, but no power to outlets and receptacles which are presumably all on the same circuit. I pulled every dead outlet as well as every live one that was adjacent or close to the dead ones and found no problems with any of the connections in any of the boxes or fixtures nor any power in the dead fixtures. The wiring is very strange in the home, there used to be two kitchens, 4 breakers do not seem to feed any outlets or lights in the home so I shut them off completely as I labeled the rest. The dead circuit (believe it or not) includes the garage, part of the basement, the boiler, and 3 gfi outlets in 3 separate bathrooms, one in the basement and 2 on the main floor. I checked each bathroom gfi, there's only one black and white in each box so that's not the problem.

After checking every possible fixture, box, receptacle, switch, etc, I figure that the bad connection is probably buried in a wall somewhere and not traceable or fixable.

So I found a nearby outlet from another circuit which is fed from a 15 amp breaker, and ran a 12-2 wire to a nearby dead outlet and all the lights and receptacles now work. They worked after only connecting the black wire indicating that the problem is definitely an open hot lead somewhere, but I connected the whites as well.

The circuit I tapped into powered only a few outlets in the basement as well as a half a bathroom and the only time the circuit tripped so far was when all the garage lights were on and using a vacuum so the amperage of the new combined circuit seems adequate.

My concern is that if the power returns to the formerly dead circuit now I have 2 circuits feeding to several receptacles and lights and also, I have effectively reversed the direction of the electrical flow to the circuit on at least some of the receptacles by tapping into one that is somewhere in the middle of the circuit that went dead.

What are the potential problems/safety risks of leaving it as is.

I have shut off 4 breakers that don't seem to provide power to anything in the home, so I figure 1 of them is the circuit that went dead but there's no way to be sure.