I highly doubt Vergil is not capable of same feat as NTDante. And if we assume Vergil's has a punching force that is 5 times weaker than NTDante's, all Vergil has to do is apply speed to it. By doing so he can probably exceed the force that NTDante made when he punched that ugly being.
Even so, his punch didn't make Poison's swing much. The force just punched her out of the edge and moved her a bit. And you don't even need that much force to get same results if you hit poison, all you need to do is hit her hard enough so that she will loose her balance. Then the weight of her body will move her when she falls.
It looks more "heavy" than what it actually is because NTDante is in motion, and that gives a sense that his punch threw Poison that far, when his first punch (uppercut?) hurt Poison head - she raised her head backwards, and then he punched her in face again.
And the rest is gravity.
Second law of Newton: Force = Mass * Acceleration
I said earlier we cud assume Vergil's force was 5 times weaker than NTDante's. But i think Vergil's force is at least 3 if NTDante's is prolly 5 without speed applied.
And i think NTDante's speed is 3, while Vergil's speed may be 6+.
Where 1 is average human force/speed.
So 6*3=18 (Vergil), and 3*5=15.
So Vergil's punch force would then be 18, NTDante's would be 15.
EDIT:
Also about "Speed doesn't matter, if Dante hits Vergil once hes finished"
I could say the same thing about Vergil slashing NTDante.
Plus here you see in this video how quick Vergil is, and hes facing Beowolf. Lets assume Beowolfs speed is 2, while NTDante's speed is 3, and they both have same force when punching.
Not only did Vergil finish Beowolf off quickly, he also punched him with such force that the corpse hit the roof brutally.
And i'd argue thats a bigger feat than NTDante's feat.
NTDante hit poison and she fell over, he didnt punch a poison laying on floor, he punched her head then punch it again and she fell over.