If your opponent makes a viking or two to hunt OLs, just replace them defend your main base OLs with queens. You should generally only have 5 or so OLs out on the map without speed and keep the majority of your OLs safe in your base.
If you aren't going broodlords and your opponent makes tons of viking, go kill him. He won't have enough stuff because his supply is locked in useless air units. If you are going broods you defend with a few corruptors and infestors.
I've yet to build infestors. They're intimidating.
Am I supposed to use a combo of infested terrans + fungals? It takes 4 fungals to kill a viking from full HP.
Infestors are arguably the best unit in the entire game (but also a bitch to use well). You would only need to kill vikings if you were going broods, but yeah you would do it takes with fungals + ITs. Mostly I fungal once and snipe with corruptors but if there was a huge group try and keep them locked down with multiple fungals and kill them however you can and you basically win.
He had like 10 vikings at his peak. I didn't see a way to attack him since my army was all roaches and he had a pretty solid siege tank line on high ground behind a wall-off - seemed suicidal to attack into it.
10 vikings isn't really all that much. The thing is, you catch them once in a big fungal, they're all going to go down provided you can continue the chain. Then you make a few corruptors (if you don't already have some, which you should unless they died) and snowball air control in your favor since he'll never be able to rebuild vikings fast enough to retake it.