Best suggestion is create a Linux Live USB stick or Live DVD such as version MINT and boot from that and use that to backup any data that you have to an external location if you have anything important on it as Linux will allow to gain access to your data.
Then place original Vista media into drive and perform a clean installation which wipes the drive clean and installs fresh, as I dont believe a repair installation will correct for lost credentials, then copy your data back to system from the backup that you created with use of Linux Live Distro.
If you dont have the original OS media to install Vista then suggestion is to either buy a copy of Vista and install that or Windows 7 and install that as installing Windows 10 on it, you might not have adequate RAM for Windows 10 to run without serious performance lag on the hardware intended originally for Vista. Windows 7 will run fine on 2GB of RAM if the system is one of the Vista machines with just 2GB RAM. If you have 4 GB RAM Windows 10 can run on it but expect lag in just about everything you do as Hard Drive will be very active and the virtual memory that is used will slow down the system as it uses the hard drive as additional memory for the system to make up for low RAM.
If Windows OS is not needed and you want to go the route of Linux OS, Linux OS runs quite well on systems with just 2GB to 4GB of RAM without the lag that Windows would have and Linux is free. Only issue with Linux is if you plan on running software that was intended for execution on Windows, there is a feature called WINE which can allow you to run software intended for Windows on Linux, BUT it usually operates at a degraded performance than as if it was run on Windows and any video games run through WINE are horrible because of lack of DirectX and use of OpenGL for graphics.
The fact that I need DirectX keeps me from switching to Linux as my main go to OS, but if you just web surf and check e-mail and are happy with free software out there for Linux then it can be a perfect replacement for Vista when your not a gamer or the system isn't to be used with games intended for Windows execution.
