The 20-year-old was released by Walsall at the end of last season without making a senior appearance.
He scored once in 31 matches while on loan to National League side Chester last season, where he played in both matches against the Tynesiders.
"I've settled in really well," Vassell said. "I've been made to feel really welcome by the lads."
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Gateshead have signed defender Theo Vassell on a one-year deal after a successful trial.