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alexwheezy
The first option will create the value of only the last element. You can check this if you write, for example,

v@P += (0,@mask*3,10);

You can see that the remaining elements have no effect on the expression.
To make it work as a vector you need to change the example a little.

v@P += set(0,@mask*3,10);

Oh, alexwheezy, thanks a lot!
It works when I use set(0,@mask*3,10);

And I've noticed that claiming a vector should use {} instead of ()
so it also work when I type:

v@P += {0,5,0};

But be carefully ,it occur an error if you reference a property @mask ，like this：

v@P += {0,@mask,0};

so in my case, the best way is to use v@P += set(0,@mask*3,10); like you say.

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