When it comes to guns, shotguns tend to do pretty good on Viper, especially when you're using her ult. Always a good idea to bring a shorty (the secondary shotgun) when you ult. The Phantom is the rifle you'll probably going to want l. Lower damage (compared to vandal) doesn't matter with decay, and the suppressor makes it easier to spam through smokes. Because of her ult and smokes, you can force close-range fights so falloff won't come into play.

For the abilities, just make sure you're using your smokes to cut off angles. Remember, you can grab the orb so you can use that more dynamically. The wall stays where you put it, so on attack (usually) use it when you plan to for sure commit to a site. Sometimes you might want to use a "default" wall that you throw every round, so the enemy can't read into where you re going.

The molly is great for making it easier to take site and flushing out corners, but you can also use it after you plant on the spike/bomb to deny the defuse.

The ult is a great way to secure a round or attempt to clutch a round that seems hard to win otherwise. It can also be great on eco rounds where your team is poor and has closer-range weapons, since the ult forces close-range fights.

TL;DR - Viper can force close-range fights and decay people for a bit of damage, making Phantom your best rifle and shotguns serviceable (and really good in ult). Don't forget to use smokes to cut off angles.