Some temporary cast ons are real tricky but the crocheted one is very easy and I use it all the time now. I think you take the auxiliary yarn out before you fold the hem up to knit them together. For an increase in this situation I often use a simple backward loop thrown over the right hand needle. Looks good to me.
For that crocheted temporary cast on you make a crochet chain and then pick up the back bump of the chain to form a stitch in. I just wanted to tell you what to look for, this is not a detailed explanation.
Your vest is beautiful. Good luck with it and have fun.