|Buttonholes in Knit fabrics used to give me the heebie jeebies. Then, I accidently came across this technique.
Cut a separate interfacing pattern for right and left fronts. The left front interfacing is as usual, cut to the fold line. The right interfacing is cut 1/4 inch or more past center front.
Fuse your interfacing as usual, but now for your buttonhole area you have a double layer of fused interfacing. This gives the buttonhole area great stability. I have used this technique so far with vertical buttonholes, on stretchy thermal knit, jersey, sweatshirt knit, etc.
I got this from a kwik sew pattern with a placket, where the entire placket was interfaced, not just up to the fold line. I dreaded those buttonholes, but they surprised me--they came out perfect with my plain ol' Kenmore. In fact, with that thermal knit, that was just about the only part of the garment that was NOT stretched out!