I see JDRoberts has some dev links to get you started. It’s really been so long now I forget what I even read, but I know it was a completely scattered, shotgun approach until I had my first device handler working for a switch 
Then iterations…
Regarding the Vantage, it sounds like you already see that not everything will have a direct mapping. I did the programming for my Vantage system, so some buttons directly map to a single load, others control scenes, some are dimmable while others are not (appliances), I have some sensors connected, etc. In the end I ended up making 5 different device types for SmartThings:
- Vantage Switch - for on/off only control mapped to a button (which may control a single load, or a lighting scene)
- Vantage Dimmer - for on/off and dimmer control of a specific vantage load
- Vantage Sensor - for the floor sensors I have by stairwells and beds
- Vantage Group - this is an odd one, just in that I have 5 special buttons programmed that basically show the status (and do control) a large group of lights – basement, main floor, upstairs, outdoors, and whole house. So if everything in the house is off except for one light upstairs, then the “upstairs” and “whole house” buttons would be “on”. Pressing either with turn off the light and both buttons to off. So it needed a little special behavior for ST for these.
- Vantage Reload - this is also a special one that is basically for a ST button that will reload (send back to ST) the current state (on/off, dimmer level, etc) of all the defined vantage switches, dimmers, sensors, etc. ST has gotten much better, but back when I was making this things were pretty rocky and ST could easily get its view of my devices’ state wrong over time – as their servers were overloaded often. So this button would reset everything to the correct current state. Only tricky part is that the ST API has rate limiting and I have a lot of devices defined
, so it accommodates that. Was a nice addition though, as now I just have ST run that button automaticallly at 3am so I know each day should be starting out right
I guess if I were you I would start out with a simple HTTP API to turn a switch on/off and then work to hook that to a custom ST switch device handler. You’ll learn a lot in that first pass. I’m not a Java/Groovy guy, but in the end it wasn’t that bad starting by copying some example code out there.
I’m not sure what Vantage system you have, but one other possible tip is that I did end up making some “virtual” buttons on the Vantage side also. Basically for things I wanted ST to do that I didn’t want a physical button on a keypad for on the Vantage side.
Well that got long… Enjoy, I still remember that winter project 
-James