In the second printing of Ultimate Magic (March 2012), Vow of Silence gave you 1 ki per level instead of what the PRD said being 1 ki per 6 levels. At the time there was some confusion as their was no errata for this explaining the change and the PRD remained unchanged (Some Details in reference posts below). I have no clue if this was changed in the 2018 softcover printing of the book, but the PRD remains unchanged from that time (and Archives of Nethys, its replacement after it stopped being updated, still has the 1 ki per 6).
Of course, since there was no update or mention of it, its highly likely that the change was just a missed misprint and a further missed clarification as they never appeared in the only 3 posts talking about to make the change/mistake right. However, since there is no official clarification, the possibility that the second printing is correct can't be ruled out (and maybe if this gets enough attention, we can finally have the answer).
Personally I do think that this could be correct as it can be highly detrimental (though I don't think it truly matches Vow of Poverty as you are essentially restricted to one magic item and not many other items). It does limit your ability to communicate significantly (including related checks) and it makes it impossible to activate magic items that require you to be able to speak, such as command word items (as I don't think you can even use magic device to bypass this as it describes you having to do an equivalent action in order to try and activate it, ie. you have to speak if you want to activate a command word item).
What do you think about this? Also, was this actually answered anywhere else?
Reference Posts:
Monk Vow of Silence Discrepency
HELP!!! Monk Vow of Silence Problem
Easy "FAQ Friday" candidate - Vow of Silence
Well, all the PRD site only update/updated based on errata (or FAQs), so anything changed without being listed in the errata (usually called stealth errata) doesn't get changed on the sites.