Hi,
Ive actually owned 2 of the above cars and had expereince of the third (sort of)
The Vectra is the one i have most expereince with, having owned 2, a 2.2 petrol Sri and a 150 CDTi elite. The CDTi was a very capable car and pulls very strongly. They are actually larger inside (or feels like it) than the omega i used to own. They do have a few reliability issues though, ranging from dual mass flywheel failures, cracked inlet manifolds, EGR problems. Make sure its under network Q warranty as i wouldnt trust a third party warranty as they will claim they are wear and tear.
I had a 1.9D bluemotion passat as a company car. Again very capable, quite nippy off the mark but lacking guts at motorway speeds. Then again, it was built for econmony and i always saw 68mpg on a run if taking it carefully. 55mpg around town was th enorm. Nothing bad to report other than again they have dual mass flywheel failures from an early age, and have been known to fail as early as 20K miles!!!
The mondeo i havent owned, but have had a volvo S40 2.0D which is the same engine. I actually traded this is in as i got the flyhweel issue. since reading up, it appears every manufacturer suffer this so its not a ford only issue (or VW or Vauxhall!).
Out of the three, this pulled the best, very torquey but laggy low down. Once the turbo kicks in, its off very strongly. Mine did tend to smoke quite a bit though. Was the worst of the three for MPG, 35 around town and only early 40's on a run.