Vista - NET USE DOS program

I've a Vista PC and an HP840c connected via USB (it has also a parallel port
but the PC has not)

As I need to print from a DOS program I've shared the printer and connected
the LPT1 port via NET USE.

A "DIR > LPT1" issued from the Command Prompt is correctly forwarded to the
USB printer (it's DOS compatible). So far so good.

The problem is that if I open a DOS application inside the very same Command
Prompt, it cannot access the LPT1 port, as if the NET USE command would have
been never placed.

The same applies if I NET USE LPT2: and change my DOS program accordingly: A
"DIR > LPTx" from the command prompt always works correctly while a real DOS
job doesn't work.

All this has always worked up to XP. I already tried all the "Run as
Administrator" combinations. Even disabling UAC doesn't cure the problem.

Any idea about why the NET USE redirection isn't available to the DOS
program as it's to the Command Prompt in Vista ?

Thanks,
Dave