Once you defeat the Elite Four, you have two entrances to the Kanto region. One is at the Pokemon League (Which I beleive is unnacessible to get to Viridian City from the left due to ledges), one is by surfing along the water in New Bark Town. Check you map once you are in Vermilion City. On the southwest side of the region, find a green square. This is the location of Viridian Forest. That is where you can find Pikachu during the Sinnoh Sound is onthe Pokegear Radio.
EDIT: This is SoulSilver, I have SoulSilver and finished it, and I don't have HeartGold.