Vienna PD is part of the Fairfax PD radio system.
It is Digital P25 and you will need a newer generation scanner to monitor the traffic.
While online scanner Apps/Site are OK, they do not allow you to be in control and I doubt Vienna would be specifically hosted.
From RadioReference.com
http://www.radioreference.com/apps/db/?sid=6957
Fairfax P25 Public Safety System
Vienna Talkgroups
DEC HEX Mode Alpha Tag Description Tag
1305 519 D Vien PD Disp Police Dispatch Law Dispatch
1307 51b D Vien PD Tac1 Police Tac 1 Law Tac
1309 51d D Vien PD Tac2 Police Tac 2 Law Tac
The Uniden Scanners are nice, if you can find a good deal on a used BCD396XT is a good option, but to be a bit more future proof the BCD325P2 is Phase 2 capable which I believe Loudoun County is currently using Phase 2.