Pokémon FireRed and LeafGreen/Vermilion City

Vermilion is a very important city in these games. It connects you back to route 2 which you couldn't get back to until now. It has a gym, fishing areas and more.

S.S Anne
There is a deck that leads to the S.S Anne ship, and you can get the HM01 there. However, you must have the S.S ticket in order to get on it (you can get it in route 25 if you haven't gotten it yet).

Gym battle
You will fight Lt. Surge in this gym. You must have HM01 in order to get in the gym. Furthermore, Lt. Surge is standing behind an electric field, and you must activate two electric switches (they look like trashcans) to open the field. Stand next to a switch and press A to activate it. The second switch is always next to the first switch. Finding the right combination is trial and error.

In this gym, trainers and the leader use Electric-type Pokémon. They are weak against Ground-type attacks and Electric-type attacks don't affect to Ground-type Pokémon, so using a Ground-type Pokémon isn't a bad idea. You can use or  (which can be caught in Diglett's Cave) to defeat them. Raichu can be tough to defeat if it manages to use Double Team a couple of times. Otherwise, if you have a Ground-type Pokémon, this gym shouldn't be too difficult to beat.

Other
There are five important things you can do in Vermilion City before and after you win the gym battle.

Trade
The first is the trade in the orange roof house above the gym.

Vs. Seeker
The Vs. Seeker is a device which allows you to fight trainers that you have fought before. You can find it in Vermilion's Pokémon Center.

Old Rod
Finally you can start fishing. You can get the Old Rod from the house next to the Pokémon Center, just talk to the fisherman there. Old Rod mostly catches low level s.

Bike Voucher
In the green roofed house above the gym there is a Pokemon Fan Club. Talk to the Chairman there and he will give you a Bike Voucher. You may take this item to the Cerulean City bike store and the clerk will give you a Bicycle in exchange for the Bike Voucher.

Vermilion's Deck
The deck may seems useless after the S.S Anne have sailed away, but it isn't. You will be able to get to the Sevii Islands from there, and if you get the special event tickets like the Aurora Ticket, you can use them in here.

Items

 * TM34 (Lt. Surge after you beat him)
 * Vs. Seeker (purple haired girl in the Pokémon Center)
 * Old Rod (fisherman in the house next to the Pokémon center)
 * Bike Voucher (old man in the green roofed house above the gym)
 * Max Ether (hidden in the corner south to the Pokémon Center)