Republic of Ireland's McGeady, 30, has played 32 top-flight games for Everton.
Vermijl, 24, made 32 appearances for North End on loan last season and joins them on a three-year contract.
Defender Baptiste, 30 joined Boro in 2015 but suffered a double leg fracture before playing a competitive game.
The former Blackburn and Bolton centre-back ended last term on loan at League One side Sheffield United.
McGeady scored 31 league goals in 185 league appearances for Celtic before joining Russian outfit Spartak Moscow and had a loan spell with Championship club Wednesday last season, scoring once in 13 games.
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Preston have signed Everton midfielder Aiden McGeady and Middlesbrough's Alex Baptiste on season-long loan deals, plus Sheffield Wednesday defender Marnick Vermijl for an undisclosed fee.