Probably best to get it plugged in to some diagnostics* and find out why the light is on, but you can check some things yourself first.
Check all vacuum hoses, make sure they are all firmly connected and have no splits.
Make sure engine oil level is correct, low oil can affect the vacuum pump.
Get a vacuum gauge and test the system, if you don't know how to do this it will not cost much to get it done at a garage.
Check the MAF sensor, either by plugging it in to diagnostics live data or by swapping it for a known good one (any friends with a zafira?).
* Vauxhall diesels tend not to connect to generic code readers, they prefer proper dealer level "Tech 2" readers or something like Op-Com. Some garages use readers which cannot connect, but they will charge you anyway and tell you there are no codes stored or, even worse, tell you the cars ECU has failed because they cannot connect!
fred, April 2012
