Two different systems with different pluses and minuses.
Vera has quite a good rules engine that runs on a PC.
SmartThings has no rules engine, which sucks. But it does have antennas for both zwave and zigbee. Zigbee antennas are smaller than zwave and typically have longer battery life, so once you’re in a mixed protocol environment you have the ability to use a number of small battery-operated $25 sensors that just aren’t available in a zwave-only system. So the solutions often are different.
But as the saying goes, “my use case is not your use case.” One thing SmartThings truly excels at is the quality and helpfulness of its developer community. 
This particular topic originally described a use case which didn’t specify the number of devices allowed. So the app solution exists. But let’s expand that to your use case: lock after x minutes in away mode while using only the information from a Schlage smart lock, no extra sensors. Is that correct?
(I use my own lock differently: I have it set to autolock, so even if power is out in the house, the door still locks itself. But my use case is not your use case. Anyway, I’m just not familiar enough with the lock data to give you a one device solution myself.)
@garyd9 is an expert on locks (he wrote a very popular lock manager device handler)–maybe he can chime in here. Or @ethayer (another locks expert) who wrote an advanced lock manager smartapp. Or @obycode or @JoeC , who each wrote popular rules engines for SmartThings.
Or of course someone else may have a lock-only suggestion. Hopefully we’ll find one for you.
p.s. One small point: my college major was computer information systems and I worked as a network engineer with mesh networks long before I started adding home automation to my house. Sophisticated zwave controllers do as little polling as possible: it shouldn’t be necessary in a mesh network. Instead, when an event occurs, as long as you store the timestamp, the sophisticated controller should have all the information it needs to run timers or coordinated event checking. But again that usually ties in with a quality rules engine.
(Vera, for example, doesn’t poll at all unless the entire network has been idle for awhile, a sophisticated and sensible technique used to limit the detrimental impact of poll messages on network traffic. Vera also automatically skips polling of most battery-operated “sleepy” devices, another sophisticated technique to improve network Quality of Service. http://wiki.micasaverde.com/index.php/Polling_Settings )
in any case, the end user shouldn’t have to worry about exactly how the timer is done, I certainly agree inactivity is often a useful rules trigger!
See