Why would a vacuum line keep blowing off my ford e350 when I give it gas
Most common reason is age, it has most likely got too soft to hold the fitting, you could put a small hose clamp on it, but if it is too soft the clamp will squish it down and it will most likely start leaking, so I would just replace the line, then you can clamp it.
Of course you might want to check the engine mounts if one is broke it could be pulling the line off as the engine raises up. You didn't mention if the engine pops back during acceleration so that won't be a concern.
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