A yoga is arrived at by addition, which sets it apart from every other reckoning in the almanac. Where the tithi measures the Moon's separation FROM the Sun, the yoga measures the two longitudes ADDED together, and cuts the resulting circle into twenty seven equal arcs of thirteen degrees twenty minutes. Vaidhriti is the arc from 346 degrees 40 minutes to 360 degrees 00 minutes of that sum. The sum climbs at roughly fourteen degrees ten minutes a day, so a yoga runs about twenty two and a half hours on average, a little shorter than a tithi, and like a tithi it can be skipped between two sunrises or carried across two of them.
The twenty seven arcs are the same width as the twenty seven nakshatras, and Vaidhriti is the twenty seventh arc of the grid in both schemes, which is why the two are often tabulated side by side. They are not the same thing and share nothing but their arithmetic: a nakshatra is a segment of the sky the Moon is physically standing in, while a yoga is a number derived from two positions and belongs to no part of the sky at all. Reading a yoga as a star group is the commonest confusion the term invites, and the shared name of the twenty seventh position, Revati, is where it usually starts.
Vaidhriti is one of the two yogas the almanac singles out beyond the ordinary auspicious and inauspicious grouping. Vyatipata and Vaidhriti are together called the maha pata yogas, and they are marked in almanacs that mark nothing else about the yoga at all: many printed panchangas carry the yoga column purely so that these two can be found. Both fall where the sum of the longitudes reaches a value the tradition treats as a break rather than a transition, and both are held to make the whole day unsuitable for anything undertaken for the first time.