Final Fantasy 7 – Can You Return to the Temple of the Ancients?

There's just too many things in ff7 to list that are missable (with many that are permanently missable) without a guide.

However, there is a nice missable items guide available if you choose to take a look.

There a few things to take into account. Vincent's is what you could consider a magic user. His stats lean towards Magic, as a result, you probably want to equip strong green or red materia on him.

HP:        Very Low (-5.2%)   
MP:        High ( 4.6%)       
Strength:  Very Low (-11%)    
Vitality:  Low (-9.1%)        
Magic:     Above Avg ( 2.8%)  
Spirit:    Above Avg ( 4%)  
Dexterity: Very Low (-10.1%)  
Luck:      Below Avg (-3.9%) 

You mention that you have Vincent's ultimate weapon, which is Death Penalty. This gun's power increases every time Vincent kills an enemy, using this formula:

Power = [Power * [Vincent's Kills / 128] / 16] + 10

So eventually, he could become really strong if you put some effort into it.

Additionally, there is a well known glitch that causes Vincent to kill enemies with one shot, provided the Death Penalty's power has been boosted to an extreme value. More information can be found here.