Have you tried defining it as / ?
What is the url you use to send a request to the virtual service?
You get a 404 not found, that means that the VSE is listening on the port that you target. If not you would get a connection refused if nothing was listening on that port. Next, the VSE looks at the incoming URI and validates if the incoming URL matches one of the basepaths of the virtual services deployed for that port. The fact you get a 404 means IMHO that the incoming URL does not match the basepath.
It's a bout root cause analysis, if you try basepath / then you should be certain whatever is send to that port ends up inside your Virtual service, and you should see the transaction count go up.
Cheers,
Danny