Hi,
A few things:
the \overrightarrow is set to stretchy, meaning that in a correct mathml implementation of the browser (…) the arrow should cover all non-space-separates chars:
\overrightarrow ABC D is equal to \overrightarrow{ABC}D
The \vec is set to stretchy=’false’ which implies only a single char will have a vector (the vector is a special one « &RightVector »…this is not an arrow !)
The reason this will not workout as expected is caused by the browser, and not the MathMl code generated by wims…
We could try to mimic the Tex behaviour more closely…but be aware that the differences between current mathml browser-versions is the cause of all « trouble »
(just try Chrome/MathJax on the same tex-code in « direct exec »)
kind regards,
joke