Troubleshooting pybitcointools transaction signing/verification
OK, I figured it out.

It was an issue with the value of v in vrs (v being the version byte, and r/s being the DER values). Since v is not encoded by DER signatures, the check for 27 &lt;= v &lt;= 34 was failing because not knowing the v value would sometimes require v be equal to None, 0 or 1.