Shadow priest Vampiric Touch dispel dmg

What is wrong with the damage of Vampiric Touch? What i mean is, that i see a lot of players who dispel my VT like its nothing. In every guide is written" Don't dispel VT/UA", but still people dispel it and as i see the penalty dmg is s**t (rarely does soething like 11k crit). I did a few tests with my priest( 3291sp and 1-ish% crit) and the results were...what could i say: in the worst case on dispel VT did 3.7k penalty dmg, and in the best case did 11k. The average penalty dmg was between 4k and 5k. So my question is: Is it something wrong with VT?