There is an excellent map at their website that shows their exact location
http://vistalinda.webcindario.com/frame.htm. Coming from the airport you would probably end up taking a slightly different route by way of Uruca rather than going through downtown but the cabbie directions would be the same whether coming from downtown or the airport. First let him know it is in the Calle Blancos neighborhood. When he asks for more detailed directions tell him "trays see-in-toes metros nortay eey kwartoh see-in-toes metros westay de Durman Eskeyvel" (for you spellcheckers, that was deliberately spelt phonetically) thats about 300 meters north and 400 meters west of Durman Esquivel, a major plastics factory and landmark north of downtown. If your cabbie doesn't know Durman Esquivel, its time to get out and find another cab. When you get to the plant you'll take a little side road to its left and go down and up a small valley. When you get to the end tell him to take a left (ees-kyarda) and go ALL the way to the very end. Linda Vista is at the end of a short dead end road where Calle Blanco ends just before it hits the San Jose-Quapiles Highway.
As for getting between the Gulch and Linda Vista, as I said before you can use the exact same directions to get back. Obviously you really don't need any special directions to get to the Gulch. Just say Hotel Del Rey. Once you know your way around better, its also very easy to catch a bus between downtown and Linda Vista. They leave every 15 minutes during the day from a stop about 100 ft from the front gates of the Linda Vista. You get out at the end of the route along with most every one else around Ave. 5 and Calle 3 which is just a few blocks from ground-zero on the other side of Morazon Park and 1 block west of the Holiday Inn. I've also walked all the way between Linda Vista a couple of times when I was feeling industrious, but it is not exactly a short walk (20-30 minutes at a brisk pace).
As for catching Interbus, probably the closest major hotel is the Radisson. Its a short walk from the Linda Vista. You take a short path through the brush down to the highway and the Radisson will be a few hundred yards down the road on the other side (be very careful crossing the highway). BTW, you'll pass the Motel Eden along the way, but I don't think you should tell Interbus to pick you up there. The Hotel Villa Tournon is also pretty close although not as simple a walk. It is across the street from El Pueblo and is along the bus route I mentioned above so you can get there easily by either cab or bus. You could also just catch a cab to the Del Rey or take the bus and have them meet you at the Holiday Inn.
Finally, if you're feeling slightly more adventurous, you can save close to $50 for your roundtrip (or 1 extra MP session) and take the public bus for $2 each way. Especially since you're probably going to have to take a cab or bus to get to your pick-up spot anyway. It may not be quite as comfortable as Interbus, but it's really not that bad and you'll get to sit with some real ticos instead of just a bunch of other tourists.