Enabling the true administrator account for login opens you up to the highest risk of attack - files are assigned to the ownership of a non-logon account for a reason. I personally disagree with ever enabling the true administrator account with net logon privileges.
Granting your own ID as authorized is the second-most unsecure (Take Ownership script). Any compromised program or web script may be able to gain access has the same administrator rights as you. I did this for non-windows files because I want access to MY DATA directly, not through library pointers; it may protect basic users, especially on multi-user machines, but many users can handle it themselves.
Turning off the UAC is basically a minimal line of defense at best. It's sole purpose is to notify you whenever an app wants admin rights - but if you are always going to say YES whenever it asks then it really is no defense at all! It is rare that malware just tries to slip in; no it offers you something, it doesn't need to slip in because you are going to open the door! You are installing a desktop wallpaper of Megan Fox or getting free anti-virus software, so you ARE going to say YES because it's something you want. They don't mention the extras you don't want and the UAC can't tell the difference; you approved it!
So will it make you a little more vulnerable, yes slightly, but as always your real defense is keeping your anti-virus up-to-date (a good active version like MSE and a passive version like Malwarebytes is a pretty good defense) and the most important protection of all is YOU! Know what you are downloading, what you are installing, stay away from dangerous sites and suspicious programs and make external or network back-ups and remove them / turn them off during normal operation. My back-up drive is off 6 days a week so if something gets to my primary drive I know my back-up couldn't have been touched.
If you were an extremely paranoid person you would create a non-administrator account and only browse the web under that ID and you would never ever download anything; but most sane people don't go to this level.