Vanquish can be great fun while it lasts
Apart from that the worst part of Vanquish is it's length, but that doesn't mean there wont be enough places to explore and enemies to kill throughout the game. The reason everything comes to a close fairly quickly is because of the fast paced gameplay. By sliding you can traverse the whole battlefield in mere seconds and by slowing down time you can precisely aim for headshots to take everyone down fast.
Don't expect much help from you allies, they are just there for show so that they can take damage and force you to come heal them and maybe get a weapon or upgrade as a reward for saving them. The enemy will still focus more on you and less on them despite you having 10 allies.
Outside of action the only thing you can do is go about upgrading your weapons by collecting more of the same once your ammo for that particular weapon is full. There are about 8 or 10 different weapons to choose from though you can carry only 4 at a time.
There will be some tricky collectibles to find so chances are you will be replaying many missions in order to find them. Apart from that the only other mode to play is the challenge arenas which you just take down waves of difficult spawning enemies in a single room.
Don't play this for the story, it's just here to give you a reason to do what needs to be done, but doing that is fun and challenging while it lasts