。
It is a most excellent thing that your curiosity is so vigorous.
Literal
Curiosity [subject-が] honorific-vigorous [nominalizer-の] [topic-は] greatly excellent thing [formal copula-である].
Formal-register sentence with keigo flourishes. お旺盛 attaches the honorific お- prefix to a 漢語 (Sino-Japanese) noun — which is actually unusual, since お typically pairs with native Japanese words and ご with 漢語 (so ご旺盛 would be more standard). The speaker is likely being ironically pompous, or the form reflects an older/elevated style. なの nominalizes the adjectival-noun phrase. である is the formal literary/written copula, distinct from neutral だ or polite です. 大変に結構なこと 'a very excellent thing' is a stock formal compliment.