Implement switching back and forth, with a sticky preference.
There is a toggle button that reverses colors (light background in VE mode, black background in wikitext) as the primary switcher.
However, there is a secondary way to switch to VE, clicking the "preview the result" links (see mockups). This is because there is no preview button, and people using wikitext may still want to preview (but we'll just send them to VE).
See T90764: S5. Flow: Integrate VE toolbar in Flow entry fields .