This is possible with AI scripting. The best way to do that is using Proud Clod to edit the SCENE.BIN. Do be aware that FFVII's AI is written in a rather awkward pseudo-assembler - if you do not have any programming experience, this might require a lot of trial and error.
To detect Vincent in a battle, you need a creature to see if any enemy has a FormationNumber of 0x16. If it does, you can then display a message with the 93 opcode. To insert Vincent's name into a message, use the code {EA0007h}.
Here's a script that will do exactly what you need. Insert it at the start of Hojo's preBattle AI:
02 20a0
01 4060
80
60 16
40
70 0044
93 "Hojo: 'Hello {EA0007h}.'"
93 "{EA0007h}: 'Hello Hojo'"
This displays the following at the start of the battle if Vincent is present:
Hojo: 'Hello Vincent.'
Vincent: 'Hello Hojo'
If you intend to change this dialog, you will need to make sure the address following the 70 opcode still points to the first instruction after the conversation. Proud Clod will do this for you if you enter the script first, *then* edit the lines.
Hope that helps. What were you thinking of having Vincent say?