Vim s-underdot

Vim accepts Unicode characters in insert mode following CTRL-v. Prefix Unicode values with u, as in Python, and enter two byte codes as four character hex. Vim ṣ is CTRL-v u1e63 in insert mode. Thanks to Mark for the tip.

Vim theta

:digraphs gives the vim table of digraphs.

And CTRL^K followed by h* gives lowercase theta.