found download from last time but cant find the serial number on my account?

If you open Vegas Pro 10 on your old computer, you should find the serial number on the splash screen as well as under the Help menu on the 'About Vegas Pro' tab.

Several months ago, Sony shut down their Sony Creative Software servers so activation though those servers is no longer possible. It looks like that also applies to retrieving serial numbers from the Sony era. The News section of this forum has a workaround that should enable Sony-era programs to be activated:

https://www.vegascreativesoftware.info/us/forum/activation-of-sony-creative-software-era-applications--149297/

... but you will need the serial number. The process self-generates a new Computer ID so there's no need to use the Computer ID on your old computer (which wouldn't work anyway because it wouldn't apply to your new computer).

At the time of the change of ownership from Sony to MAGIX back in 2016, serial number data should have transferred to MAGIX, and if I remember correctly, access to the MAGIX My Account was via the same username and password that was used to access the Sony My Account.

BTW, it's always a good idea to keep a copy of all software serial numbers either via print outs, in a notebook or on some form of password protected database.