I've taken Vicodin a few times, none as serious as your dad's but I'm gonna toss in my 2 cents, and some good thoughts for your dad. I have taken it for back, dental (root canals and wisdom teeth) and 2 knee surgeries. The only time it ever got to me was the 2nd knee surgery - I was in a total moody funk. Probably a depression - but never diagnosed. At times I thought it was the Vicodin but I also chalked it up to other things.
The surgery was expensive, I had ins, but I still had to pay for part.
My knee hurt like all get out and I thought it would never stop
I couldn't sleep bc my knee hurt, even w/ the Vicodin
Work stress was big at the time
PT hurt like a b........
I couldn't sleep bc I hurt
PT was slow goings at first and I thought it would never get better
Lots of Vicodin (2 every 3 ro 4 hours) was making me sleepy
Did I mention I couldn't sleep?
Living on caffiene bc I couldn't sleep
Missing workouts bc of knee surgery - missing w/o puts me in a funk
I actually almost feel asleep driving on the highway to work a few times, and one morning hit a curb near work bc I was so tired and hurting I couldn't focus. That made me mad and sad and feel kinda hopeless too.
I never had any problems w/ it any other time, but the 2nd knee surgery was the longest stretch I've ever taken Vicodin.
So, I don't think the Vicodin itself is causing the depression, but the funny, foggy, groggy, funk feeling from lots of pain killers is probably not helping any other issues. I imagine he is scared and he hurts, and probably isn't sleeping well. Is worried about his finances, his wife, his kids, his estate and all that.
I think its a bit of everything.
Best wishes to you and your family, and I hope it all turns out ok
Nan