The Virbank Complex is split into two parts: Inside and outside. Both of them have different items and Pokémon, so both are unique when looking for things.
The outer area contains a few things, but the meat is really on the inside. Firstly, there is an X Accuracy by the sign, so pick that up really quickly for a free Accuracy buff in battle. Then, there is a Roughneck standing at the bottom of some stairs by the water. Talk to him and he will trade one of your Poké Balls for a Great Ball, which, like he says, is a really good trade. Keep going through this path for a Silk Scarf. This is an item that boosts the power of Normal type moves the wielder uses.
In the grass, you can find some new Pokémon. First off is Magnemite. Magnemite is an old favorite, and is very useful. Electric and Steel is a really good type, boasting 13 resistances/immunities in this game. When it evolves, you can get stuck in a predicament. You can keep it as a Magneton and use an Eviolite to boost defenses, or fully evolve it to a Magnezone for the raw power. Either way, you have a good Pokémon on your hands. The only drawback is the double weakness to Ground, but that can be overlooked.
Next up is a pair of version-exclusive Pokémon, both useful in their own merit: Magby for Black 2 and Elekid in White 2. Both of these Pokémon evolve into somewhat-mixed attackers, but they require trading to reach their full potential. Elekid evolves into Electivire, which is a really good Physical-oriented mixed attacker with an amazing movepool, and Magby evolves into Magmortar, which is a Special-oriented mixed-attacker, with a just as amazing movepool. Breeding these Pokémon gets them other nice moves, but they can work with what they are given for sure. However, be aware that their catch rates are fairly low, so bring a few Great Balls with you.
Immediately in the inside area, there is a man. By talking to him, he asks if you can get some workers working again, because they took a break and never came back. The three workers are located on the thin walkways above, so you have to take the stairs up there and navigate to them. Do not stay on them too long, though, because your player character will lose balance and fall off of the walkway. You have to defeat all three of the workers for it to count as you getting them back to work. One is right in the obvious across a walkway, one is to the left of the first one across another walkway, and the last one is south east of where the first one was. Next to the third one, though, is TM 46: Thief. Thief is a physical Dark type move, with 40 Power and 100 Accuracy. It steals the held item of the target, but the output is nothing of worth, as you will be getting a more powerful one later (albeit Normal type, and not a TM). In addition to the workers, there is a scientist who is talking about the components about the complex. After following him and talking to him three times, he realizes you are interested in what he has to say, and give you an Ether.
After talking to the man at the beginning after defeating the workers, he gives you TM 94: Rock Smash. Rock Smash is 40 Power, 100% Accuracy Physical Fighting type move, with a 50% chance to lower Defense. You will need this in the aftergame to do a couple things, but aside from that, it is pretty useless.
If you fill the habitat list of the area, then a girl in here will give you five Great Balls, but you can not finish it unless you trade due to version exclusives and some Pokémon found in the water in the first part. However, there is a hidden Great Ball in the empty patch close to the Youngster. Also regarding hidden items, there is a Tiny Mushroom at the empty spot in the dark grass by the Scientist's starting position. This Tiny Mushroom actually regrows occasionally, so you can potentially pick up more and more of them for selling.
At the entrance of the complex, go left and you will see a Cut tree. You can come back later and cut this. Behind this is a Super Potion, which is something you can't really pass up.
New Pokémon on the inside of the complex are actually older favorites, Growlithe and Koffing. Growlithe is a Fire type that fulfills multiple roles on a team. Even competitive players are not quite sure what role its evolution Arcanine specifically fills or the potential of this Pokémon. The trick to using a Growlithe is to not evolve it until it reaches level 46, because you cannot teach Arcanine moves that only Growlithe can learn. You can evolve it and reteach it any moves that it would learn as an Arcanine, however. Overall, it is a very good Pokémon and should be used wisely. Koffing is a wall. Poison types are built to be walls, and Koffing defined this since Red and Blue. It has really good defenses, and decent offense. However, walls are not easily trainable, so that is a warning if you choose to train a Koffing. Luckily, Audino can be found in this grass, and they still give off a lot of experience.
After getting Rock Smash, there is not really anything else to do at the complex, so the only option left is to challenge the Gym.