Yes you would lose your Vista bootloader if you reinstalled XP. Over at Pro-Networks they have a couple of apps liek VistaBootPro that might be able to recover it. But i dont think it is from a full uninstall and format. I think that is just from a Repair.
Maybe this info i just found might help you out.
1. Right click the My Computer icon/Properties/Advanced/Performance/Settings. Verify that: Use visual styles on windows
and buttons is checked.
2. Windows Explorer/Tools/Folder Options/View. User Common Tasks in Folders.
3. Right click the Desktop/Properties/Appearance. Windows and Buttons choose: Windows XP style.
4. Start/Administrative Tools/Services. Scroll down to Themes and verify that the service is started.
Go to Start/Administrative Tools/Services/Themes. Set to Automatic. Also check your settings here:
Start/Administrative Tools/Services/Terminal Services/Log On. At the bottom ensure that Hardware Profile Original Configuration is Enabled.
If the above doesn't apply/nor help: Only windows Classic is listed under Appearance:
1. Right click on the desktop and choose Properties.
2. On the Theme tab, choose the Windows XP.
3. If this option is missing, you may choose Browse... to C:\Windows\Resources\Themes\Luna.theme
4. The Luna.theme is the theme of the Windows XP.
If Luna.theme is missing: Go to Start/Run/Msconfig/Expand Files: Luna.MS_ and Luna.TH_
Restore Luna Theme (Line 187)
Restore Themes Functionality (Line 187)
Windows/Buttons Greyed Out under Appearance and/or Windows XP Style Changes to Windows Classic or (Line 214) http://www.kellys-korner-xp.com/xp_tweaks.htm.