Blu,

Should the shirt color match the socks? (off white)
Off white will do nicely, especially if you already have a pair handy.

Would it have been formal enough to have worn a matching black vest with the Argyll as father of the bride?
Depends on several factors, including what time the wedding is, and what the groom's party is wearing. If it is a day wedding (before 4-ish), then I would suggest the Argyll jacket. At an evening wedding, a Charlie would be more appropriate, but then again, the old rules about no black tie before 5 pm are "oot the windae" anymore anyway. (Hamish, what say you?)

Should I button the vest and leave the jacket unbuttoned?
I don't like to see buttoned Argyll Jackets, and Charlies do not button. Show off the vest -- do you have a pocket watch & chain? Always a nice touch.

Done properly, a belt should not be worn with a P.C. and vest - true statement?
One or the other -- waistcoat or belt.

Should I leave the sgian dubh home that day?
Not especially, unless you're concerned about trouble with the local constabulary. I suppose you could check with the padre/ministed officiating, but I wore my sgain dubh to my wedding, especially since it was a gift from my wife.

Can anyone here enlighten me with regards to the use and traditions of the quaich?
Tradtionally, after a toast is given, the quaich is drained, flipped upside down, and the bottom of the quaich is kissed to signify that it is empty. "Slainte!" (Slanje) is certainly appropriate to end the toast.

Cheers and congrats!

Todd