Ok, I've experimented around a bit, and here's what I found out.
  First up, I wanted to make sure that there wasn't anything wrong with my discs, so I installed the game on my laptop which runs XP. Everything worked fine, I used the original game installer and downloaded the patch, which applied without any issues and the game started perfectly. 
 Then I used the launcher/installer from bgbennyboy, installed the game without any problems but when I was going to install the patch, I got the same error as on my other computer.
 Grim Fandango is already up to date.
 The game data is more recent than this update. 
 The game still starts without a problem though. 
 So, if I use the launcher/installer, I cannot apply the patch normally, neither on Vista nor on XP. 
 My guess was that it had something to do with the registry, so I did a little trial and error on the entries there, and found the problem. There is a dword named "MAGIC" and it was set to 00016382 for me. When I deleted this parameter, the patch would install without a problem, both on Vista and XP.
 After the patch was installed the "MAGIC" parameter appeared again, with a different value (00092774). 
 And as for my other problems, I don't know if I should blame it on Vista or 64-bit OS or both... 
 So, to sum up!
 To get Grim Fandango to run on my 64-bit Vista system I had to:  
 Install the game using bgbennyboy's launcher
 Remove the "MAGIC" dword from the registry
 Install the patch (using the launcher or manually)
 Launch the game :D  
 One more thing, Vista has a funny way of putting stuff in the registry, it doesn't put the Lucasarts stuff under [HKEY_LOCAL_MACHINE\SOFTWARE\, but instead under [HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\.
 More Vista magic :lol: 
 Jona, your DVD installer sounds interesting, I look forward to trying it :D 
 :thmbup1::thmbup1::thmbup1: Silvek - many many thanks for your solution! If fortunatly must be added to GF tools desined by 
http://quickandeasysoftware.net/) and others!!! But... we are still awaiting for GF REMASTERED for PC then!!! ))) 
 LucasArts Quests like Rock'n'Roll - they 'll never die! ;) 
 z