Cara Delevingne -- Best Wedding Plus One Ever ... Just Ask GF St. Vincent

Cara Delevingne and her musician bae St. Vincent, took their relationship to the next level -- doing the wedding date thing, and it looks like the best decision the indie pop rocker ever made.

Cara & St. Vincent (real name Annie Clark) have been dating for more than a year, but finally went public when Cara recently referred to her as her gf in an interview. Perfect timing too, since the couple jetted off to West Texas last weekend for a wedding.

Based on pics from the event ... Cara was the life of the party, as usual. St. Vincent's friend/makeup artist was the one getting married, so Cara was the "plus one."

She wears it well.