You can extract seeds from the grapes themselves! Under "Farming" in the crafting menu, there is a recipe called "Gather Seeds". You need a nearby built herbalist table, a knife and the grapes of course. This way you get grape seeds!

There are a few fruits and flowers you can get seeds from this way. My guess is that certain "seeding plants" dont give seeds, but instead they yield more if you wait longer for them to grow.

Oh, let me try that! Thanks