Bug in vertical vectors: ∪ˡ makes a new entry

[williamdemeo]

Description

This address issue #482

Vector elements with curly braces and union symbols were split (at the union symbol) into two vector elements. This PR fixes that.

Checklist

• [X] Commit sequence broadly makes sense and commits have useful messages
• [X] Code is formatted according to CONTRIBUTING.md
• [X] Self-reviewed the diff