Few details would be needed- assuming its a multipoint then it will at least start on petrol and switch to gas once its warm enough to be off cold start enrichment (effectively the equivilent of the choke on an older vehicle). So until the coolant has reached whatever temp is set in the lpg ecu (which depends what the installer set) it will use petrol to run. This wouldn't be the case with a single point, but I'd be expecting a multipoint on a Vectra really.
Its possible you have petrol leaking somewhere - a failed petrol injector being one possible place where you wouldn't be able to see it happening, this could cause overfueling that the cars petrol ECU will try to correct but probably be unable to do much about.
Any idea what system is fitted? If you have one of the newer systems you might be able to access diagnostics to investigate by switching cylinders off to see what happens. Advice will depends on what you are using, posting photos of the parts you can find will help. Or if you can find the ECU for it then what it says on the ecu probabbly much more helpful.
I assume by adding petrol to the tank it hasn't sorted the drivability out - does it run ok on petrol?