I was asked to write a victim impact statement (VIS) for an assault case that occurred a few months back. I wanted to know if it's worth writing it? It's a minor assault case where I was left with a bruised mouth. No weapons were used.

I received death threats from the accused (online and by a mutual friend) unless I drop the charges against him. Is this appropriate to be included in the VIS? I also had to pay for the ambulance bill. How likely will these points affect the accused in the sentencing?

This question is fraught with the ability to turn in two very different directions.
A bruised mouth can be a deadly warning or it can be a minor scuffle after too many drinks in both of you.
It can be an abusive man hurting women in an escalating pattern, or it can be two equally strong men arguing.
It's tempting to jump right in and say 'Of course you report the death threats' but maybe caution is best unless people know the whole story, including just exactly what the threats say. And what were you doing in an ambulance if it was so minor? There's something strangely off about this.

It was a random attack by the accused. No arguments, nothing. I was walking and got attacked from behind.

I wouldn't have known if the injuries to my mouth were serious or minor unless I called the ambulance, right? I had to take X-rays of my mouth and was notified by the doctor that no treatment was necessary and it should heal within days. At the time of my injuries, I thought it'd be serious as I was unable to speak properly.

At first it was hard to tell if you were indeed hurt a lot and afraid to say so.. keep in mind, no one here was there, and no one here knows you. We don't even know your respective ages or your gender.

I am not sure why calling an ambulance is required rather than getting yourself to an ER. (I would have gone to a dentist instead. Teeth can decay rapidly after a blow.)
Anyway, just me personally, I would drop the charges.
Not because of intimidation or fear.

You CAN'T drop charges. Only the DA can drop the charges.

You can refuse to provide a statement, refuse to testify, see how far the DA wants to take it.

- but you cannot drop the charges.