What it is
Vimshamsha divides each sign into twenty equal parts of 1°30′. As in the shodashamsha, the starting point depends on the sign's nature (movable, fixed, dual), but the starting signs are different.
How it is calculated
Rule: in a movable sign the first part is Aries, in a fixed sign Sagittarius, in a dual sign Leo; then onward in order.
Example: a planet is at Aries 17°. 17 ÷ 1.5 ≈ 11.33, so the twelfth part. Aries is movable, so the twelfth sign from Aries: Pisces. In D20 the planet is in Pisces.
What tradition looks at in it
Parāśara links the vimshamsha to worship. Tradition looks here at the kind of devotion, meditation or service a person's mind settles into. Jupiter and Ketu, and the lords of the 9th and 12th houses, are looked at. It is not a test of faith; each person has their own path.
How much the birth time matters
On average the ascendant moves about 1 degree every 4 minutes; some signs rise faster, some slower. One part of D20 is 1°30′, so a difference of roughly 6 minutes in birth time can change the D20 ascendant. The Moon crosses one part in about two and three-quarter hours. In your chart, ePandit shows next to every varga how many minutes of birth-time change would shift its ascendant. With any uncertainty in birth time, it is wiser not to build on the D20 ascendant.
How to read it calmly
Read D20 as a second look, not a verdict. Tradition first studies the 9th and 12th houses and their lords in the main chart (D1), then the running dasha. D20 only helps check whether the same theme shows up here as well. A planet's sign in this varga decides nothing on its own. Spiritual life is a matter of the heart; this varga does not rank anyone's faith.