According to prevalent definition of Vashi Yoga in Vedic astrology; if in a horoscope, there is one or more than one planet in the house prior to the house of placement of Sun, Vashi Yoga is formed in the horoscope. Vashi Yoga may bless the native with good results related to money, wealth, prosperity, authority and many other things.
The placement of Moon, Rahu and/or Ketu only, in the house prior to the house of placement of Sun is not considered for the formation of this Yoga. Hence one or more than one planet other than Moon, Rahu and Ketu should be placed in the house prior to the house of placement of Sun in a horoscope, for the formation of this Yoga in that horoscope.
In real practice, Vashi Yoga is not seen producing such results when only the aforementioned conditions are met. Hence more conditions should be met in order for this yoga to be formed effectively in a horoscope. If Sun is functionally benefic in one such horoscope, this yoga may produce better results. If the planet/planets placed in the prior house are also benefic; Vashi Yoga may produce even better results.
Taking an example, suppose Sun is placed in the second house of a horoscope in Aries and Saturn is placed in the first house in Pisces. Both Sun and Saturn are malefic in the horoscope. In this case, Vashi Yoga may not be formed in the horoscope, though the primary conditions are met.
Taking another example, suppose Sun is placed in the second house of a horoscope in Sagittarius and Jupiter is placed in the first house in Scorpio. In this case, Vashi Yoga may be formed in the horoscope, since both Sun and Jupiter are benefic. Hence the native with may witness good results related to profession, reputation, finances, health, wealth, family, spiritual growth and a number of other things.
Once the formation of Vashi Yoga is confirmed in a horoscope, the next factor to check is its strength. The strength of this yoga is calculated through the placements of planets involved in its formation in various signs, nakshatras and navamshas; as well as through the influences of other benefic and malefic planets on them. The overall theme of horoscope and running times (Mahadashas) also affect the strength of this Yoga.
Taking an example, suppose benefic Sun is placed in the tenth house of a horoscope in Leo, in Aries navamsha of Magha nakshatra and benefic exalted Jupiter is placed in the ninth house in Cancer, in Cancer navamsha of Punarvasu nakshatra. Sun and Jupiter are strong in their respective signs as well as navamshas. Hence strong Vashi Yoga may be formed.
Looking at the running times; when Mahadashas (planetary periods) of malefic planets are in effect; benefic effects of Vashi Yoga may reduce. When Mahadashas of benefic planets other than the ones involved in the formation of this yoga are in effect; benefic effects of this yoga may increase. When Mahadashas of the planets involved in the formation of Vashi Yoga are in effect; the native may witness maximum benefits through this yoga. For instance; in the recent example, the native may benefit the most during Sun Mahadasha and Jupiter Mahadasha.