The problem is not the weapon sets. This is the problem -
http://www.ddovault.com/cgi-bin/yabb2/YaBB.pl?num=1555475678/0#0If you are heavily invested in the VKF tree with intelligent choices in supporting feats, none of the other piercing weapons in the game will be better than Pain + Suffering. If you built for daggers + VKF and Pain+Suffering isn't the best DPS weapon set for you on trash and bosses, you've built poorly. Conversely, if you're using daggers and didn't go deep into VKF, daggers in general and even Pain+Suffering is a bad choice.
Outside of rogue-based builds that can double-dip on dagger specific bonuses from both the assassin and VKF trees, daggers are shit compared to the other weapon options.
It's possible that there are a few very specific non-rogue based builds that can make daggers work well, but those will be rare. Maybe Barbarian VKF or a Kensai VKF? But even then, you'd likely get more DPS of the same concept but built for non-daggers instead.
The steadily increasing amount of W available on good melee DPS builds with non-dagger weapon types is increasingly putting daggers at a disadvantage. W is a multiplier. Multipliers benefit weapons with higher base damage die more than ones with low base die.
This is why the OG Duality handwraps were so stupidly overpowered when Ravenloft first came out. Monks could already innately increase the base damage die of wraps enough to keep them competitive with other types of weapons. Then Duality came out and allowed monks to end up with something like a 24W or 27W multiplier on a D10(?) base damage die.
Think about a W(1d10) vs a W(1d4) weapon. With a 5 W value, the avg hits would approximately 27 vs 12, for a difference of 15 on average. With the right bonuses, slightly better crit profiles, etc., it's not too hard for a 1d4 weapon to make up a 15 point difference.
Up the W value to 10 and it becomes 55 vs 25, for a difference of 30 on the base damage. Suddenly, you need twice as much from the right bonuses, crit profiles, etc. to stay on par.
Up the W value to 20 and you get 110 vs 50, for a 55 point difference. Now you need even better bonuses on daggers to remain competitive.
The bonuses from VKF will only get you so far, even when stacked with the assassin tree. On anything outside of an assassin and a few specific, odd builds, weapons other than daggers would be better and easier to build for.