A vacuum leak should be fixed. Your job is to locate the leak. At hot
idle carefully listen for the leak's hiss. You'll probably find a
cracked or disconnected vacuum hose, probably close to the engine rather
than in some remote corner. The hose can be temporarily taped until you
get a replacement, be sure its vacuum hose.
The reason to fix vacuum leaks is that the leak allows unmetered air
into the engine making its air / fuel ratio too lean for a smooth idle.
While driving at higher speeds, a small leak becomes a smaller and
smaller percentage of the total air entering the engine so the effect is
imperceptible to the driver.