maybe change to 1 mana and give +1/+1

Seems pretty complex and wordy for a common.

its not that complicated, two of the lines of text: flash and enchant creature dont mean much. It's a combat trick that is also a payoff for the toughness subtheme. Maybe I could remove vigilance, but I like how a combat trick giving vigilance makes you think about whether to cast it before or during combat to catch your opponent off guard or to have your creature untapped.

The general red-flag rule for text amounts is 4+ lines at common. Given this is a common, in the limited environment it means that the players should become somewhat aware that at any point an opponent having 1 white mana up potentially means they can give a creature +1/+1 and have it deal combat damage equal to its toughness rather than power.

Being aware of the common combat tricks is pretty critical for any competitive limited play. In the case of one of those combat tricks being this, that adds a lot of (often unnecessary) combat match evaluations for opponents - and some for the white-color player playing opponent as well in the case of bluffing having the combat trick (ie. representing it as if you had it even if you don't).

Then also as a common you have to think of the scenarios where this card is showing up in multiples relatively frequently. That is most certainly going to give the combat math that non-NWO angle with extra board complexity that has the potential to lead to all kinds of bad scenarios such as analysis paralysis.

There are currently only two common cards ever printed with "use-toughness-instead" abilities, one which was from modern horizons and another which required the creature to have/get vigilance from an external source (or for the creature have it) for that particular ability to be active. It's definitely red-flagged, dunno what your red-flags stats are currently (>20% is ok).

The wording btw should be like:

> Enchanted creature assigns combat damage equal to its toughness rather than its power.

do you think it should be moved up to uncommon?

It would IMO be a good candidate for an uncommon upshift, yes.