Yes you act as if the spell was a spell known. I do not however think as written you could retain the metamagic from a scroll that already had metamagic on it.
IE
You have a scroll of shield. You can cast shield using the vest as a level 1 spell. You could use your extend spell feat to cast it extended as a level 2 spell.
However say you have a scroll of extended shield made at caster level 20. You do not gain the benefits of the caster level or the metamagic, you only act as if the shield spell is a spell known.