Update values after placing an order

[Siguana]

  J3 and VM3

  I need to update some values just after placing an order, but I don't know which file I should edit.

  Which file should I edit?

[jenkinhill]

Editing an order?  http://docs.virtuemart.net/manual/general-concepts/220-orders.html

Or maybe you want something else, in which case please say exactly what you want to do.

[Siguana]

  I need to modify the code of the file just after the line where the order is saved into the database.

[Jörgen]

This makes it not much clearer. What code do You want to change ?

Jörgen @ Kreativ Fotografi

[Siguana]

  I just want to add some values in a custom table that I created.  The values must be inserted after the order is placed and before sending the mail to the seller.

[Milbo]

Then you need to write a plugin, which uses the plgVmConfirmedOrder trigger.