Learn Portuguese (Portugal): Vamos começar a auditoria amanhã de manhã. - We are going to start the audit tomorrow morning.

No, it is a completely different word! When you start an action, you use the preposition a followed by an infinitive verb (like começar a trabalhar). But here, you are starting a noun. The a before auditoria is just the feminine definite article meaning "the".

Since our current lesson is about ficar a (to end up doing/knowing), how would I use that here to say "We will find out about the audit tomorrow morning"?

You would combine the future structure with the periphrasis: Vamos ficar a saber da auditoria amanhã de manhã. The phrase ficar a saber (literally "to stay to know") is the most natural way to express acquiring new information or "finding out" about something in Portuguese.

Why use the structure vamos começar instead of the actual future tense word começaremos?

While começaremos is grammatically correct, the true future tense sounds quite formal and is mostly reserved for written or journalistic Portuguese. In everyday spoken Portuguese, expressing a planned future event—especially one happening as soon as tomorrow—is almost always done using the verb ir plus an infinitive, exactly like vamos começar.

Why do we say amanhã de manhã instead of translating "tomorrow morning" without a preposition?