i believe all of the vampires that were originally printed "2 hand damage" have since been errata'd to "+1 Strength," so yes. A Vampire with +1 Strength striking with a Sengir Dagger will do two aggravated damage.
www.secretlibrary.info/ is a great resource for looking up current card texts, as well as the cardlists in the download section here at VEKN.net.
Some vampires have X strength, not +Y strength, eg. Call the Great Beast has 4 strength, or a vampire playing Torn Signpost has 2 or 3 strength.
Anyway, it doesn't change the result.
Name: Sengir Dagger
Cardtype: Equipment
Cost: 2 pool
Unique melee weapon.
Strike: strength aggravated damage.
If a vampire with 2 strength strikes with the Sengir Dagger, the strike deals 2 aggravated damage.
Prince of Paris, France
Ratings Coordinator, Rules Director