Considering the Red Legion took down the City in a single attack (and the first time the Cabal in general even come to Earth) when they arrived, being able to fight the Vex on Mercury and win would've been pretty easy for them. Heck, they even took Mars effortlessly and destroyed the Black Garden portal, effectively ejecting the Vex from the planet.
The Vex more than likely tried to stop the Cabal from invading Mercury but could not stop them, let alone even trying to take the Almighty.
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The Vex can beat the Cabal, hands down...no contest. The Vex are playing a war of attrition. They hold the Cabal, and let their resources and soldiers dwindle. Why overwhelm when you can watch the enemy waste away? Whatever the Cabal destroy, the Vex just replace. The Cabal don't have this capability, and will thus take the L over time.
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In the long run yes, but we're only talking about the time needed to detonate the Sun. If the Red Legion were capable of overwhelming the Vex on several planets in a very short amount of time, then they would be able to keep pressure on the Vex long enough for the Almighty to finish it's job (and judging from how much of Mercury was eaten in the short time Ghaul was around, it would've taken a few months at most).
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1. There is one instance wherein it appears that the Cabal have overwhelmed the Vex, and that is the new exclusive Playstation strike. In all other instances - at least all the ones I can think of - the Vex are either holding the Cabal at bay, or are not bothering. 2. The Vex don't care about the sun, and in their ideal future, it has been extinguished. 3. Perhaps the Vex no longer care about the Infinite Forest, as it has been taken over by Osiris. Thus, they no longer care about Mercury. I know the Almighty occurred before, but the Vex- being the Vex - probably saw this coming, and just used the Infinite Forest temporarily for some computational power. Or have an isolated instance before guardian arrival to run tests.