minsyoo, regarding fishing the Vedder River, you can certainly use your spinning rod for casting spoons and spinners. While float fishing with roe is effective, it is not necessarily more effectively than spincasting. That being said, I love float fishing on the Vedder River. Watching the float dives just after dawn is an addiction that is hard to shake off.
The 8ft rod that is rated 12-15lb test is fine, especially if you connect with a few chinook or chum salmon. The 6'6" rod should be good for coho salmon, but it would be a big challenge if you connect with a chinook or chum salmon on that rod.
Personally, I just bring a rod for float fishing and do not focus too much on spincasting when fishing the Vedder River, but at times I do bring a spinning rod with me too.
My spinning rod for the Vedder is a 6'6" rod that is rated between 8 and 12lb test. I use 8lb test line to accompany that rod, because I primarily target coho salmon.
12lb test on your 8ft spinning rod is more than enough to handle most fish. 15lb test line would result in more difficulty in casting your lures and is not as enjoyable to fish with in my opinion.
Regarding float fishing with roe or other bait and presentations, as Mr P has pointed out, a rod that is 10'6" or longer is more appropriate. A long rod is needed to keep the line between your rod tip and the float away from the water surface so you can get a better hookset when a fish bites. You can use a spinning rod for float fishing, but feeding line to the float as it drifts downstream becomes a frustrating challenge with a spinning reel. A baitcasting or centerpin rod outfit is more appropriate for this type of fishing.
For more information on float fishing on the Vedder River, take a look at this article:
http://www.fishingwithrod.com/articles/2003/1003_01.htmlHere's another one that you may find helpful:
http://www.fishingwithrod.com/articles/2004/1104_01.htmlGood luck.