If you go to the trouble of getting LPG i reckon (1) only do it if you're gonna keep the car for a while, (2) only fit gas injection (not the old school mixer setup), and most importantly (3) the installation is done but someone who knows what they are doing.
A VX will run fine on gas and i recommend it as long as (1) (2) and (3) apply. Doesn't burn out valves if installed properly - that's rubbish!
There's three types of LPG systems:
1. mixer (old school setup) - actual usage is around 30-35% more than petrol - cost approx $2400-$3000
2. vapour injection - around 20-25% more than petrol - cost approx $4000-%4500
3. liquid injection - around 10-15% more than petrol (don't believe people who say only 5%, that's sales bullshit) - costs approx $5000-$5500
On the first two you CAN fit either the standard tank between the strut towers OR a donut tank in the spare wheel well - don't let any installer tell you you can't. AFAIK there's no donut tank for liquid injection yet (as the tanks are higher pressure) but i may be wrong.
Being pre-Jan 2004 you can fit any if these three to your VX (after then you can only fit gas injection due to the legislation etc).
The old gas mixer setups need constant tuning (every three months) and go out of tune very easily (i.e. start backfiring & blowing airboxes, poor consumption, rough idle, etc). The newer gas injection systems don't have this problem (they don't go out of tune as such) and never backfire like the mixer setups can.
I had a mixer setup on my VT supercharged 2007-10 then 3 months ago had a front end kit fitted (coverted) to vapour injection, that cost me $3000 as they reused the tank & lines etc. The original old mixer setup cost me $2400 in 2007.
My vapour injection installation is a Sequent Plug & Drive system (an aftermarket version of the VE's Sequent system). Vapour injection has it's own injectors for each cylinder. One minor thing i realy like about my system is that the trip computer fuel usage figures are accurate on both fuels, something most aftermarket systems (including my old mixer setup) won't do.
Vapour injection runs and idles better, you don't lose traction control, and it revs harder. Starts on petrol then automatically switches to gas at 39 degress, two cylinders at a time. The changeover is seemless. Performance-wise i can't tell the difference on either fuel and i don't drive like an old lady. I couldn't say that about the mixer setup though, it was noticeably worse.
I don't reccomend you get Liquid injection right now. Yes it's more efficient than vapour injection but it costs a lot more and has only been around for a couple of years. It'd be nice to have but honestly vapour injection has been around longer, and most LPG installers are familiar with it. Liquid injection is the future, but just right now IMO.
Good luck with it.
