I would recommend get them out in the van with you. I found I really enjoy the company and actually get bored working on my own now! After 6 months or so i think you get a good idea as to whether they're gonna stick it and whether you can trust them to work on their own.
One van one van is definitely more profitable but when you think about the costs of buying and running an extra van, and putting a whole new set up in there the advantages are shortened somewhat. obviously if expansion is your aim then you have to do it, but if you don't ever want more than one employee than i would suggest just stick to one van.
Similarly, although this is just an assumption, staff are more likely to get fed up and leave if they just work on their own every day and window cleaning can be extremely boring when its not your own business.