There is no syntax to make something a pointer, just a few bits that pass by reference.
You can pass an object/class into a function, and it passes by reference. From there you can target it's properties and update them. Whether or not this is ideal practice, you'll have to judge for yourself.
If you want to completely avoid constructing a new vector as well, you can update it's x and y values.
Here is example of these two things.
var vec = Vector2(1,1) setget update_vec
func update_vec(x, y):
vec.x = x
vec.y = y
var thing = MyClass.new()