I don't know of many patterns that do write out both sides. Basically it means do the BOs and decs at the other edge of the front.
For the L front you probably bound off a few sts at the beg of a RS row for the underarm, then decreased right above them. For the R front you can't BO at the end of a row, so you need to BO on the WS row to put the underarm at the other edge. Then you can dec at the end of the RS rows instead of at the beg.
sue- knitting heretic
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This means that instead of decreasing on a right side row (for example), you'd decrease on a wrong-side row so that you get two mirror-image halves of your vest.
For beginner sweater patterns, I recommend Patons "next steps" books. You can often find them for about $5 at Michael's or JoAnn's. Book 2 is for pullovers (and they have vests) and book 3 is for cardigans (which also has vests). All the patterns are rated "easy".