Vayamos is the conjugation of the verb ir that matches the first person plural (nosotros - we <em>in English</em>) in the subjunctive mood. We use the subjunctive to express desires, doubts, the unknown, the abstract, and emotions. Subjunctive sentences often have parts linked by a relative pronoun (que, quien, como). Here we use the relative pronoun que. Therefore:
<em>La señora nos dijo que </em><em>vayamos</em><em> al museo de oro.</em>
because its less expensive than the taxi and driving your own car, also she like viewing the streets and the subway will be underground so she won't see the streets and countryside.