Vector.new(0, 0, 0). Just plain 0 means we eventually try to add 0 to a Vector, and since Vector doesn't support addition with a scalar, it falls back to a normal addition operator, which tries to convert Vector to a Num.
Thoughts: I don't see any obvious way to figure out the correct "zero" to extend the coefficients with. Nor do I see any obvious way to allow you to add 0 to a Vector. (At least without opening up general vector plus scalar math, which is a bad idea IMO.)
It also suggests that it may be a very good idea to go beyond implementing the operators that work to implement dying versions of the operators that shouldn't be allowed, because allowing Perl 6 to fall back to its own operators will produce less useful error messages. (Or worse, they might even "work".)
I can see one more potential error like this lurking in the