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Cognito ergo non precor!
 
Not really, is it? Rex mortus [est is implied], vivat [optative] regina. Seems perfectly fine to me.

The est cannot always be implied (Vulgar Latin is for the illiterate). You have the right idea about vivat, however there is no such thing as an optative mood in Latin. It is a hortatory subjunctive. The tense of vivat is rather unclear because one should not you two different tensed verbs in two separate clause which are in the same sentence. Additionally, I prefer to put exclamations after hortatory subjunctives.

My correction:
REX·MORTUS·EST(.)
VIVAT·REGINA(!)