Could you provide a few other bits of info?
“Media Info” – most post the “text results” which can be selected in the application. Use this tool on your source media, and report the results.
https://mediaarea.net/en/MediaInfo/Download/Windows
Inside Vegas timeline or media window, right click and select properties on your source media. Goto the general tab and post the media info here, specifically looking for the Plug-in information.
Since you installed the K-LITE pack, I would:
Uninstall KLITE
Uninstall Vegas
Use CCleaner Reg cleaner to clear any bad entries
Use Malware Bytes to scan/clean any remaining crapware, and reboot.
Install all MS updates under windows update, and reboot if upgraded.
Verify your GPU driver is the most recent, reboot if upgraded.
Reinstall Vegas and test your output again.
Use something like GPU-Z to verify that your GPU is in use while rendering to MC Encoder with Cuda.
I would do all that after you post your info on your source media.
Also Under the MC Encoder, choosing VBR and using 18Mbs Peak, and 12Mbs Average, 4 slices, and enable progressive download.
Rendering Sony AVC is a good idea too, your profile would be the same as your project settings with about 10Mbs, 16Mbs, or 22Mbs bitrate. YouTube HD is only 8Mbs, but uploading more and allowing YT to toss the rest is preferred.
You could also try rendering to XDCAM-EX and uploading that file to YT.
Hardware wise, I would not think you HDD is holding you back, unless you are attempting to work with uncompressed video sources. You could use something like Speccy, under storage and verify that your HDD MAX transfer mode and Used Transfer Modes are the same. Then under Windows Device Manager, under IDE ATA/ATAPI controllers, verify that the SATA controller is listed as a AHCI controller. With out AHCI your system will not take advantage of certain drive performance enhancements like NCQ.