What stitch seam for 2x2 Rib sweater side seams?

Hey ladies, I have been working on this sweater vest since Thanksgiving – it’s seems like forever! I am finally done except for the collar. I have blocked and am ready to seam up the sides…
What seam stitch do you experts recommend for the side seams, the body is done in a 2x2 ribbing.
FYI – The pattern is from Knit & Crochet Combined, "Winter Vest"
Incidentally, I won’t even be able to wear it until next winter since it’s already warming up here in Katy TX!
I used Bernat Cashmere Natural Blends in Coal and the collar yarn I chose is a Debbie Bliss Cashmerino Astrakhan. The collar pattern is the Astrakhan stitch in crochet (never done it – cross your fingers for me).
What do you think?
Oh one more thing – I just blocked the vest last night with steam and OH my goodness it sure turned out nice…for some reason I am never good with actually doing the blocking thing even though I hear you MUST do it – now I know why and am so impressed…

Thank so much for your advice!

I use overcast and mattress stitch. Go to my site