I'm going to assume you're looking for a Steam game. Well going through the list of stuff I have on there have you tried Recettear? You sorta build up your own little item shop and can play it extremely casually to semi-casually, since usually when you go item hunting in a dungeon with an Adventurer you want to clear 5 floors of the dungeon and depending on the size that can take a bit. Not to mention that you can just open up shop and haggle with people if you don't feel like committing "that much time", although I don't think these sort of things take long at all, but I never played Plants vs Zombies so I don't know how much commitment that tower defense takes. Also has a demo version, although that one seemed harder than the actual game, but it allows you to get a taste of the first ingame week or so.
Another thing you could look into is something like Terraria it sort of allows you to build up stuff and even make your own little town, and it has some boss fights. I got some enjoyment out of that game, but I don't feel fully confident recommending it either. I guess it's upside is that you can play it with friends and to be honest that's the primary reason I had it, because people talked me into buying it. I never really understood the minecraft craze nor will I ever like that game, but at least Terraria seemed bearable to me so I bought that to play with my friends.
Aside from that maybe some sort of turn based strategy game, in which you can build up a village or something? I don't really know, I don't have much experience with these sort of games, aside from Heroes of Might and Magic (3 and a bit of 4) and that one isn't all that focused on building some sort of town.
I tend to gravitate towards the sort of games that want you to invest a lot of time into them so I can't really be of that much help. Also I wouldn't say that these two games are good on the level of Fallout New Vegas, so I wouldn't expect too much out of them. I just tried to mostly stick to what the post said.