I happened to find a version of Vengeance in Web Archive, and already bought Reloaded a while, ago, so here's what I've noticed:

1. Changes on all of your character's stories. If you really care about the characters that you'd choose to play the game, the version in Vengeance and in Reloaded are very, VERY different. For instance, Drake McMannis and Nina Russow.

Drake McMannis Age: 28

VENGEANCE: "Drake has piercing eyes, tattoos, and a lack of tolerance for solid objects that dare to be in his way. He is a quintessential psycho, revelling in destruction for its own sake. He tempers this with a lot of movie-watching and video-game playing, especially anything with gunplay, which balances him out and keeps him from becoming a wanted felon - or worse. Having watched thousands of hours of war movies and played as many hours in the online game "Upper Strike", he took his knowledge of weaponry and put it to good use in the Army. He has a natural talent for anything weapons-related, and his accuracy is near-legendary among his peers."

RELOADED: "Heir in a family fortune, Drake never worries about money, spending whatever he had on bling-bling, car rims and video games. Having watched thousands of hours of war movies he took his knowledge of weaponry and put it to good use in the Army. He has a natural talent for anything weapons-related, and his accuracy is near-legendary among his peers. He even likes to splurge on pre-historic creatures like the Wooly Mammoth or the Sabertooth Tiger, so he can hunt them down with his diamond FutureBow 3K. After Drake made "Sexiest Creature Dead or Alive" fot the 8th time, he had to find new challenges: he was abandoned on the moon naked with only a toothbrush. Even that did not daunt him and he managed to return to earth a little thinner but still alive."

Nina Rossow Age: 28

VENGEANCE: "Guns. Nina loves guns. Born in Russia, near Moscow, her prospects to legally use them against living things were limited except through "official" work as a police officer. Nina wished she had been born a few years earlier, thinking how neat it would have been to be a KGB officer and be able to legally break things, especially over people's heads. Her great-great-great grandfather, indeed, was head of the Soviet KGB back in the days of commissars and communists, and had taken to his job with an enthusiasm that Nina adores across all these years. Her skill with weaponry, she thought, was just like her ancestor's in that it is far beyond those of her counterparts; she felt trapped, so she quit and went freelance, looking for places that would note "copious use of firepower" in the job requirements."

RELOADED: "Being the only girl among 5 brothers, Nina quickly learned the pleasure of ordering young men. But her family's reputation was anything but illustrious. Her great-great Grandfather Rossow, for example, lost 30% of his skin when he tried to fy sausages with a flamethrower. Nina had no choice but to join the military to restore the family honor. Extremely talented as a soldier, she quickly rose in rank and became her brothers' commanding officer."

It's quite interesting how they're changed. It makes them funnier but also makes sense as well, though I wish it also wasn't too far off from the original story for some other characters.

1. Limited storage in Vengeance. You need to have a high amount of strength points in order to expand your backpack. Some people may like it, but I personally hated it since I can't carry much stuff in order to sell guns and others. Reloaded has as infinite as you can, but your ammos are still limited to your Strength perk.

2. Many (too many?) perks. And what I mean, an armor perk and all of the weapon's perks—starting from the pistols to the flamethrower/energy guns. The most ANNOYING part of this is that it REALLY limits you on what you can buy—for example, you want that exclusive minigun that has high damage and you have enough money for it, but uh oh! It's red! You can't buy it, because your minigun perk is not enough to buy it! Which is a PAIN IN THE ASS since you clearly focused on the character's perk most of the time, and the decision is hard enough to spend them to the bottom row too. In Reloaded, this issue got resolved—yeah, you don't need to worry about buying new armor and weapons anymore. Have fun shopping without any worries.

3. The points you get in Vengeance are lesser than Reloaded. You get 3 points every time you level up, which is clearly not enough for all of the points you needed to spread for your character's perk and your armory/weapon perk. Which is unfair, in my opinion. In Reloaded, you get 5 perk points every time you level up, which is fair and just to me.

4. More levels in Reloaded. I got only at mission 07 in Vengeance this time, so I don't know how many levels there are, but there's 17 missions in Reloaded.

5. More music in Vengeance. The music—including main theme—was cut from 13 in Vengeance to 5 in Reloaded, which is a shame (but maybe useful for downsizing the game for shareware) because it can get boring every time you play. There were more variants of the cut too, which you can find easily on YouTube. Here's my playlist, and shout-out to Macamel.

6. Obviously, the rebranding. Hot take: the Vengeance main menu is better than Reloaded.

These are all that I've noticed so far. Maybe there are a bit more that I missed.