By definition, two vector spaces being isomorphic means that if $h$ is their isomorphism, at least, the following is true
where $a$ is an element of the field and $x$ is a vector. This means that $a$ should be in both fields. But think about vector spaces whose fields are isomorphic, but not equal. Then the structure of the VS (vector space)is not changed and, hence, should be something like isomorphic. Then what property do these VS have?