2008 Vauxhall Vectra intermittant oil warning light

I am not aware that there is a "low Engine Oil Level" warning light on a Vectra (unless it is a High Spec Model).
If it is the light with the "Oil Can" symbol on it, this relates to "Low Oil Pressure" and you would need to get the fault read and checked out.

If it is a light with an "Oil Can" symbol over a "horizontal line" that relates to Oil Level but as I said above is only in a High Spec Vectra. If you have such a Vectra and you find the Oil Level is ok then equally you can get the fault read and corrected.

Hi , sounds like a faulty sender unit - Unless you are running the engine on the wrong or cheap grade oil , as this can set the oil level sender off , Or goes that thin when heated that it cant support the pressure required to work the warning switch .
When did it last have an oil change ?
I recently put Asda semi synthetic ( £16 a gallon ) into my zafira dti , it lasted 200 miles before the oil lamp started flickering when at idle .Changed it for Castrol semi and problem solved .Incidently the Castrol was the same price from a local wholesalers ....