Typically constraints are set up before simulation. However, you can also set up constraints dynamically that occur during simulation. If you dive inside your Vellum Solver, you can create Vellum Constraints nodes and wire them into your
force_output to create all kinds of effects.
For example, you create a Vellum glue constraint that will glue points once they get close together.
Set the Constraint Type to Glue.
Change the Create Constraint parameter to Each Frame so it will check to see if there are any nearby points every frame.
There is a maximum Constraints Per Point parameter that is set to 1 by default, so there will only ever be 1 constraint created.
You can optionally turn on Breaking and set a Threshold where the glue constraints will separate.