I wouldn't go for vinyl as it will peel way too easily, there are iron on t-shirt graphic printable sheets you could use that go on with a very low heat, if you iron it on REALLY low with a protective cover it should be ok, better to try a test patch where it wont matter as much first though.
apart from that i'm not really sure, stencils and a polymer spraypaint? you're right you might find some stencils on ebay but they might not be to your liking, most stencils tend to turn out "blocky". keep in mind if you spray anything with a petroleum base it may damage your kite's coating.
you'll probably need a primer to stop any ink running, but hairspray should do the job for that.
I don't really know how you would go about printing a kite since the only experiences ive had are with art and design and a little graphics, but nothing printed on ripstop.