I havnt had a problem with adhesive not bonding using this method before

We found the best way is to use a black pigment in your contact adhesive before spraying we usually use the pigment that we use for fiberglass resin depends on what glue you are using ,also a powder dye in your contact adhesive could work and spray on very lightly on the vynil

in all honesty it took me a while to get used to using it I would recommend getting a guide foot for your double needle machine ,dedicating threads for the double needle once it is set up and you have the hang of it , it really will save you a lot of time ideally you could get your French seams done in half the time it is a great investment you will never go wrong by having extra machinery this will ultimately make you more efficient time equals money in our trade and well if you can do a French seam with minimal effort like on the double needle machine then why not

We use a typical gc20606 double needle walking foot it makes the job way simpler especially for double stitched diamonds