Yes, you will have to take them for life post op. My team recommend any good quality complete tablets. So centrium, sanatagen, severn seas etc. Also tesco and adsa;s versions are very good. I was just told about having to take vit D too, I take tablets from tesco at the moment, but will get the spray when I run out. In addition to this, most people will need vit B12 supplementation post op. I take a sublingual tab now, but a lot of people have injections every quarter. Also iron levels need to be monitored, and may require you to take tablets (or in severe cases, IV infusion)
It is vital that we have regular blood tests (min once a year) to check all our levels, as deficiencies are manageable and in a lit of cases avoidable. The way I look at it, there are thousands of people walking around with deficiencies doing all sorts of harm, and dont even know about it. Because we have these ops, we get looked after and monitored well.
To answer your other questions, as time progresses after the bypass you are able to eat more. Your pouch and the join to your jed heal and they stretch a little. This is natural and right. So that eventually you are able to consume a 'normal' and healthy quantity of food. This means you will maintain your weight. However, this also means that you have to make the right food choices long term, as we bypasses can regain significant weight.