Summarize:

Vinyl is remarkably easy to keep clean with a simple rinse, if you do it often enough. Unless there is mold or mildew, or tough/thick stains, some high-pressure water should be more than enough. For single story houses, a garden hose with an attached nozzle should suffice. For multi-story houses, you'll likely not reach without a pressure washer, which can be rented cheaply at most hardware stores. Wind will pick up dust and dirt that may stick to the walls, and slow-drying vinyl can leave streaks. Neither are serious issues, but if you work with the weather you'll have an easier, neater job. You'll be much more effective if you work in small areas, which will help you keep track of what you've done. Furthermore, you want to rinse the walls off before they dry completely to get rid of the dirt for good, which will be impossible if you try to tackle too large of an area. You don't want to wash the walls from the top down, as this will tend to lead to streaks of dirt instead of a clean wall. If using a pressure washer, do not use the highest setting -- keep it low and gentle. If you have "slats," the stacked horizontal bars of vinyl, you want to try not to force the water between the layers of vinyl. As you get higher up the house, the hose will be hitting less powerfully, and this isn't as much of a concern. Still, when possible, blast at a downward angle, or straight on, to prevent loosening the vinyl siding in any way. Work in the direction of curved areas, corners, or molding. Again, you're trying to prevent water from getting stuck or lodged in openings in the siding. If left in dark, cool areas, this moisture can quickly turn into mold or mildew. You want to be careful at any junctions or areas where you can get under the siding, and you want to be especially careful near flimsy screens or windows when using a high-pressured hose. Consider using a long-handled brush, or removing the screens and scrubbing with a cloth and water from the inside of the window. These same rules apply near wiring, plumbing, and gutters as well. At the very least, lower the pressure considerably before proceeding. The first run will dissolved and dislodge the dirt and grime, and it will stay wet as water from above trickles down. Your rinse, which should start from the very top, then removes all of the dirty water to leave your house spick and span.
Use a high-powered garden hose for small houses, and pressure hose for larger walls. Plan to work on a sunny day without a lot of wind, if possible. Work on only one wall or section at a time to prevent streaking. Start from the bottom, running the hose side to side as you move up. Spray at eye level whenever possible, preventing water from pushing in between the siding. Work way from the corners, shooting water away from any gaps. Consider cleaning around windows and doors by hand. After you've working completely up from the bottom, rinse the whole wall from the top down.