Not that familiar with the Futural 787 as I don't think they really sold them in the US. I have seen them before and they are interesting as IIRC, they have the cool looking wishbone style rear seat stays that merge into one tube before it hit the seat cluster. Not sure if the lower model number (787) means it's a lower spec/grade tubing used than the 979s. As that's how Vitus usually denote tubing spec hierarchy. It could mean that the tubing is just thicker gauge or seamed, if that's even possible with aluminum. I suspect the difference would be minimal anyway.
I would think that the tubes are bonded with glue, but this bike also came out around the time when Peugeot was experimenting with "dry" bonding methods called "Pechiney" where they did not use any glue at all on the frame and the tubes were mechanically locked into the lugs with pressure. So it is good to confirm whther the 787 did not use this same method as Peugeot had to recall all their bikes made this way (Peugeot models Comete and Galaxie) and Vitus and Peugeot shared bike building technologies/methods at that time.
What's good is, like the 979's I have not really heard of 787s suddenly falling apart, so most likely you are safe as long as the frame does not look to have been abused.