Did 'fornication' ever mean vaulting?

Yes, fornication once did mean arching or vaulting in English. The OED has it dating from 1703 in Richard Neve's The city and countrey purchaser, and builder's dictionary: or, The compleat builder's guide:

Fornication, In Architecture, is an Arching, or Vaulting.

The etymology shows it coming directly from Latin. The entry isn't marked as archaic or obsolete, but it is marked as an 'Architecture' term, so not really in common use.

However, the meaning of 'sexual intercourse between a[n unmarried] man and an unmarried woman' is much older in English, dating to before 1300 in Cursor Mundi: a Northumbrian poem of the 14th century:

Þis sin [lechery] has branches fele..fornicacion es an.

The OED also has a 1303 quotation from Robert Mannyng's Handlyng Synne:

‘Fornycacyoun’ [ys], whan two vnweddyde haue mysdoun.

This comes via Old French from late Latin.