Believe me they will survive very well with cutting WAAAY back -- and being overweight is a very ssrious health risk in cavaliers as their heart must work so much harder, and they already have valves likely to give out early because of MVD. There's a finite life in a heart valve, so if it works harder, it fails when the cavalier is younger.
So stick with the vet's directions.
I have had a grossly overweight small cavalier that came in at 8 kg at start of June and she is now down to 6.6kg. She only gets about a fourth to a third of a cup of food daily, supplemented with veggies. Only veggie treats with rare exceptions.
In the Caring for your Cavalier section of the Library
http://www.cavaliertalk.com/phpBB2/viewforum.php?f=9
there are hints for good low cal treats and check the articles on obesity and cavaliers:
http://www.cavaliertalk.com/phpBB2/viewtopic.php?t=465
Many cavaliers may initially not eat veggies but if you introduce them mixed well into their food and as treats you'd be surprised what interests them. These are MUCH better for them than commercial dog biscuits which are only empty calories and mostly grain/filler; very little to nothing that is healthy in them. If you must give them, get the smallest ones possible from a pet shop not the supermarket (the latter tend to carry only larger sizes) and break them in half and only give a single treat a day -- maybe half in the morning and half before bedtime.
Veg is a great treat as they cannot really digest it for the calories and they are low in calories anyway -- but they provide great chewing satisfaction and make them feel fuller (note: a cavalier RARELY is full and will overeat itself to gross obesity. They are well known as one of a handful of breeds with serious obesity problems and thus should never be free-fed, for example). Mine particularly love slices of carrots, frozen green beans (while frozen! A nice cold treat), broccoli stem slices, green pepper slices, banana, chopped cooked broccoli or mixed veg (great for a healthy 'filler' when you are feeding small amounts of kibble or whatever). Those mini carrots are loved by many dogs.
You can dip the veg pieces in a bit of beef broth or chicken broth to get the dog started on them if she's really not interested initially.