Test Install of 7 on USB disk then upgrade of vista to 7

Why not use the Windows Upgrade Advisor first?
https://www.microsoft.com/en-us/down...ils.aspx?id=20
Generally Win7 can run what Vista did but drivers are always an issue with older hardware. And be sure to get the same version of Win7 Upgrade as what the installed Vista is, either 32-bit or 64-bit, to avoid having to do a clean install. Also note that sales of Win7 have been severely decreased if not outright stopped, may have to hunt the 'net to find a place to buy. I have a couple of older WinXP Notebooks that can't be upgraded but are running Linux Mint just fine, better than just throwing them away.

I'd just do the Upgrade of Vista on there to Win7. No point in struggling to install it to USB which is nearly impossible and pointless to boot. A simple guide to a successful in-place upgrade - Windows 7 Forums. You be better off spending that time to Clean Install Windows 7 if she prefers 7, or reinstalling Vista following these same steps to get a perfect Clean Reinstall Windows 7.

As to the wifi, have you rightclicked the wireless icon in system tray to Troubleshoot it? If that fails plug in by wire to enable Automatically deliver drivers via Windows Update (Step 3) in Vista, CHeck for Updates and install all Important and Optional including drivers. If this doesn't provide the driver import it from the PC's model's Support Downloads webpage using the Vista driver if necessary, or google the wireless device's make/model for a Win7 driver. If necessary you can trace an unknown device's driver using Hardware Identification

Yes. That's because I thought that 7 would give a little boost to that old laptop with vista that I wanted to upgrade.
Also, I saw an upgrade as an easier path. but as AddRAM said, may be it would be nicer to do a clean install and reinstall her program and data ?

As for the usb install, I already done the first step which consist at pre-installing all windows file on usb hddd. Last step is to boot it.

So, now I wonder I do an upgrade or a clean install.

I don't think she cares at all to have 7 or vista, in fact.

but a clean install is a lot of work to reinstall all the software already installed

It's less work than trying to install to a USB drive for no good reason you can explain, and has a point: You'll have the best possible install of Win7 one can have, and it will stay that way for just as long as you stick with the tools and methods in Clean Reinstall Windows 7.