Here are several websites that owners have found useful or very useful. If you know of any others that you have found worthwhile please email me the address and I should be able to incorporate them into this list.
A possible source of information is to search for, say, 'Vectra forums' if you own a Vectra into Google or whatever search engine you prefer. This tends to bring up quite a list of useful sources.
Think carefully about your car too - many manufacturers have links with others in the industry or share the development costs on specific parts which both manufacturers then utilise. As an example - one may not think that a site for VW Golfs could have any direct interest for the owner of a Ford diesel Galaxy, but it might in a certain circumstances - the diesel Galaxy shares many parts with the diesel VW Sharan which also uses some parts which are common with the VW Golf.