From memory the standard secuity system on the VR is quite advanced.
Dual tone horn siren (3 in total)
Engine imobiliser
Alarm zones for all areas incluiding bonnet entry and boot.
Dead lock facility on drivers door
Security coded radio
(much the same right thru to the VS etc which is what i have).
I wouldnt bother installing another brand of alarm, as GMH have managed to do an alright job as factory equipment., (personal opinion).