Warbreaker: How could Vasher do this?

Question

I just finished reading Brandon Sanderson's Warbreaker. One thing I don't get though:

Why could Vasher stun Denth in their final fight by giving him his last Breaths?

In the Epilogue it is stated that

both Vahser and Denth were one of the Five Scholars who experimented with BioChromatic Breath 300 years ago. So why was Dent stunned when he was given a few Breaths, he must have had some to survive as long as Vasher. The same goes for Arsteel who Vasher killed with the same trick as he mentiones just after he killed Denth.

Answer 1

We get some idea about this from the text when we re-read it:

Denth froze, then stumbled. Fifty Breaths fled from Vasher’s chest and surged into Denth’s body. They would be unwelcome, but he couldn’t turn them away. Fifty Breaths. Not many.

But enough. Enough to make Denth shake with pleasure. Enough to make him lose control for just a second, falling to his knees.
...
“Nobody ever expects it,” Vasher whispered, stepping forward. “Breath is worth a fortune. To put it into someone, then kill them, is to lose more wealth than most men will ever know. They never expect it.” (Chapter 57)

We know that 50 breaths is what is required to reach the First Heightening, which would give Denth Aura Recognition. That in and of itself wouldn't do much, but 50 breaths is a sizeable amount.

Vivenna had at least 600 breaths given to her as she had Perfect Color Recognition (Third Heightening). We also know that she could sense how many breaths people had. Throughout the book, we see no evidence that Denth had more than just 1 at a time, let alone a large enough number for her to notice. He in fact seemed to prefer to have less as it made him less noticable to those with Aura Recognition (Chapter 35 in the cellar). How he could have a store and pull out only 1 at a time, I don't know, but he was one of the Five Scholars, so we don't know all that he knows.

Even though he had had large quantities of breath in the past, it wouldn't matter if he hadn't held a large amount in some time:

“It’s shocking, when you first get it,” Vasher said. “It’s usually not too bad if you take the Breath back after only an hour or so. Wait weeks, or even a few days, and it’s like taking it in for the first time.” (Chapter 43)

So, assuming that Denth avoided having large quantities of breath to avoid attention, we can conclude that he would get quite a shock from those 50 breaths. Imagine the amount of endorphins you have in your body, and imagine if someone injected 50 times that amount into you. I'd imagine you'd shake with pleasure for at least a moment, allowing someone to kill you if they wished to.

Answer 2

It's not outright stated, but Vasher gave Denth a lot of Breaths. And at certain thresholds, Breaths create Heightenings, some of which alter your perceptions.

My straightforward speculation is that Vasher threw enough Breath into Denth to give him Third Heightening, which alters your color sense (see the table on Sanderson's website here). Denth seemed to be a practical fellow and wouldn't keep a vast amount of Breath in him, knowing there were simple ways to acquire some and not having Vasher's scruples (or possibly, like Vasher, he stashed some in an Awakening he could easily get to). So a dump of Breath into him would flash him through the Heightenings. The overload of sensation would have thrown him. He may be used to it, as old as he is, so the adjustment may only have lasted a second. A second was all Vasher needed.

It could also be the shock of what was done - Breath is a valued commodity. As noted by Stormie in comment, throwing something priceless at someone is a good second's distraction. Breath transfer is normally a more dignified process, not something weaponized in a fight. Denth wasn't the type to be easily distracted, however, so I would rate that as a second distant possibility.