how to get vector3 from (part0.Position - part1.Position).magnitude - 1 ?

I have two parts welded together. I want to make them closer together. I can get the point where I want part1 to be by doing newpoint = (part0.Position - part1.Position).magnitude - 1, but how do I turn that into a vector3 for the new weld?

If a and b are positions, then b - a is called the displacement from a to b.

The displacement is the way to get from a to b. You travel by using +. Thus,

d = b - a

y = a + d --  == a + (b - a) == b

A simple way to go nearly there is to go, only, say, 90% of the way. You can just multiply the displacement d by 0.9 to get that:

y = a + d * 0.9

Thus if a and b are 20 studs apart, d would be 2 studs from b.

If you wanted to be exactly x studs from b, you could solve that out.

Call m the distance from a to b. Then (m - x) / m is the scale you want. m is just the magnitude of the displacement. Thus (d.magnitude - 1) / d.magnitude is the scaling factor to use:

y = a + d * (d.magnitude - 1) / d.magnitude

Notice that there's d / d.magnitude in there. That's just d.unit -- it's d but only 1 stud long:

y = a + d.unit * (d.magnitude - 1)

This makes sense. You travel (d.magnitude - 1) studs in the direction of d.unit from a to get to nearly b.

A shorter way to say this would be to start from b and go 1 stud back:

y = b - d

which is the same as

y = b - (b - a).unit