Just found this forum, and hopefully my knowledge will be helpful to some.
We have been repairing inflatables since the 60's and recommend the following for Zodiac PVC inflatables.
Some of this has been mentioned in previous posts.
Clean away any old flakey glue.
Use MEK to soften up the fabric, this also helps in cleaning away any oils, contaminents.
Using a paintbrush, apply a thin coat to both surfaces with Bostik 1669 adhesive. Let dry fully (30 mins). Apply a second coat to both surfaces and again allow to dry fully.
Press both fabrics together and apply gentle heat (glue will activate at around 28 deg C). Once cool repair will be at 80% strength. 100% will take an hour or so.
Note: there is no need to roughen up PVC fabrics. This is only done when repairing Hypalon craft, as hypalon is a rubber based fabric.
I have created an online guide for repairing PVC inflatables. If need be I can put the link in.