If we are to define a vague term, the definiens will need to be vague in exactly the same way as the definiendum is. But it is exceedingly improbable that the contextual profile of the vagueness of the definiens would exactly match the contextual profile of any complex definiendum that we could practically state, or maybe even that we could state in principle.
For instance, suppose we're trying to define "short". Now, "short" has a certain contextual vagueness profile which specifies, perhaps vaguely, in what context what lengths do and do not count as short and in what way, Either there is vagueness all the way up or at some level we get definiteness.
Suppose first that at some level we get definiteness. For simplicity, suppose it's after the first level of vagueness. Then for any context C, there will be precise lengths x1 and x2 such that anything shorter than x1 is definitely short, anything of length between x1 and x2 is vaguely short, and anything longer than x2 is definitely non-short. These precise lengths will be some exact real numbers determined by our actual linguistic practices—which things we've called "short" and which we haven't. It is exceedingly unlikely that we could construct a definiendum which will make the definitely/vaguely/definitely-not transitions in exactly the same spot. Suppose, for instance, we define "is short" as "has small length." Well, small will have its own vagueness profile, defined by a different set of social practices. It is exceedingly unlikely that this vagueness profile would exactly correspond to that of "is short", so that the exact point of transition between being definitely short and vaguely short should be the point of transition between being definitely of small length and being vaguely of small length.
Suppose now that we have vagueness all the way up. Then we're going to have arbitrarily long predications like "a is vaguely definitely vaguely vaguely vaguely definitely definitely vaguely definitely short." And which such predications apply to which objects will be determined by our complex linguistic practices surrounding "is short". It is, again, exceedingly unlikely that our complex linguistic practices surrounding some other term, like "has small length" would in every context match those of "is short".
For exactly the same reason, except when the users of one language self-consciously use a term as an exact translation of a term used by another language, it is exceedingly unlikely that we could find an exact simple translation of a vague term from one language to another, and for the same reasons as above, a complex translation is also unlikely. For we would have to exactly match the vagueness profile, and since the social practices underlying the different languages are subtly and unsubtly different, it is very unlikely we would succeed.
It may be worse than that. It may well be that no two people have the same vagueness profile in their homophonic terms, except when both defer in their usage to exactly the same community. And they rarely do.
In practice, when translating and giving dictionary definitions, we are satisfied with significant similarity between vagueness profiles.