Hi Gold,
Each house can hold 30 degrees. Mercury has moved 29 degrees in sagittarius & Sun just moved out of this house & placed at 1 degree in next house (capricorn).. thou they "appear" to be in different houses, they still are in close proximity & hence mercury is combust.
Venus has moved 17 deg in Capricorn whereas Sun has moved only 1 deg. So, Thou Venus & Sun appear in the same house, they are NOT in such close proximity & hence Venus is NOT combust.
But as far as I know, Mercury is exempted from this combust factors in astrology as it'll be close to the Sun in most of the charts. Someone correct me if I am wrong.