I am so sorry about Sam. I went through this same thing last year with my Taz. As for the image of him sleeping at the vet.....it's just that. He is at peace and no longer in pain and is sleeping.
Reading your post just brought back all the emotions from when I let Taz go. After the doctor gave him the shot to go to sleep he let out a long sigh. The vet assistant said he was finally resting. (Sorry, have to stop to wipe my eyes.)
The story Karen posted of the Rainbow Bridge is a great way to help ease that pain a little. I often just imagine Taz up there playing with the other pets I've lost along the way.
Do not let go of those memories you have. They are also important in the healing process.

RIP Sam