Now three 'volumes' of Vihari-Lala Mithra's (vlm) translation of Yoga Vasishtha have been published on Project Gutenberg. The rest is expected to be published shortly.
This has been a long project - about 15 years!
Admittedly, the translation has its flaws, but it is the only complete English translation.
Vol. 3 (of 4), Part 2 (of 2) has already been published some years ago:
On Gutenberg, you can download the files in various formats, including epub and TXT, and you can read it online (HTML).