would engine oil or vibration compromise its operation?
Not automatically.
Servos are fairly sturdy things. It's not a given that the firewall is going to expose the servo to any more or less vibration than other places in the a/c. Truth is, you need to balance your prop and build in a secure engine mount and reduce vibration successfully for the good of the a/c and if done properly, the firewall isn't going to be bad for vibration anymore than anywhere else would be.
And they're not advertised as fuelproof, but they're pretty good about being pretty much buttoned up. And there are waterproof ones available. All of them have exterior materials that're usually very resistant to fuel. So take a look at your other models. Are the insides of their cowlings fuel soaked? marked with fuel spots? what? Make your decision about how much fuel the servo will see from that.
I've never needed to put a servo that far up front, but wouldn't worry a bit about it on my models. And if I ever needed to, I'd do it in a heartbeat. And if you're still worried about it, put a simple bulkhead around it. Or put just a couple of "walls" "around" it. I've been filling my cowl areas with "pressure bulkhead" for years to route the cooling air past the engines and that offers good opportunity for shielding. That is, if you really think shielding the servo is needed.
I'd say the only drawback to it would actually be in reaching your rx with the servo connection. Seems that'd be the biggest problem with the idea.