What’s the current consumption of the TouchScreen? Do you have a spec? Assuming it’s not more than a couple hundred milliamps, you can power it directly from the 5V (VIN) pin, or VCC pin if the screen requires 3.3V operating voltage.
It’s possible to plug in both 24VAC and a microUSB cable to power RPi, but this is not recommended as having two regulated power sources would cause them to compete with each other. So if you plan to power using a microUSB cable, I suggest that you simply remove the on-board PTC fuse to cut off the path from the DC-DC converter to RPi. This way, RPi is completely powered by microUSB cable, while 24VAC will still power the sprinkler valves.