You're absolutely right, of course, in stating that "vymlouvat se" is the imperfective form of "vymluvit se". I see two useful observations here:
1) the WR Czech-English dictionary doesn't routinely give the "other" (imperfective and perfective) pair for every verb, and some verbs, by reason of their meaning, exist only in their imperfective or perfective form, and with some verbs the single form is both imperfective and perfective.
With přemluvit, domluvit, zamluvit and omluvit, for example, the imperfective forms are not given either, and with other verbs not in the *mlouvat / *mluvit family, such as brát, the perfective "pair" (vzít) is not given either, and that's the case for the vast majority of other verbs in this dictionary.
2) In some cases - and vymlouvat se/vymluvit se is one such example, though there are many more - the meaning and English translation actually change, depending on the context and how the verb is used. Vymlouvat se would very often mean "to offer excuses", "to try to talk one's way out of", while vymluvit se would be "to make the (specific) excuse that ...", and these (sometimes quite subtle) changes of meaning can only be illustrated by using the verb in specific contexts, which is something a dictionary doesn't have the space to accommodate.
So, in brief, yes, you make a fair point, but here it's a matter primarily of consistency across the WR Eng/Czech and Czech-Eng dictionary.