Hello Nicolas,
You are correct, the maximum packet length would be 1ms for the parameters you listed. This is a limitation of the 2k sample pattern memory. There is no way circumvent this or work around this to create a larger contiguous pattern.
It is also not possible to externally trigger the pattern. The VSG25A cycles through the buffer continuously. If you are creating your own custom file, you have the ability to specify a period up to 64k samples. With the period parameter, you can tell the device how long to wait before cycling over the pattern memory again.
Regards,
A.J.