All versions of linux are more secure than windows if you observe normal approved operating procedures, and can open Office files if you have the latest version of Openoffice installed.
Whether it is the best for you and worth it to "update from vista" depends on what you want to use it for (high end windows games---not going to work) or all the other computer uses, (no problem).
Before installing I would suggest that you get the live-cds for various linux distros to see which one will work best on YOUR computer hardware. Some versions of linux have problems with some graphics cards (nvidia seems to work best) and wireless cards. The live-cd will not make any changes to your hard drive, and it will run a little slow because it is running from the cd and not the hard drive. Once it is installed it will run like all your other programs. If everything works with the live-cd everything will work when installed. If for example your wireless won't work with the live-cd if you install it it will need to have some work done (look for and install new drivers, edit configuration files, etc). It CAN usually be fixed but it will take a little work and for a person new to linux it can be very frustrating.
When you find one that works and you like I would also suggest that you read the instructions and tutorials from the website of that distro on how to install, setup and use linux. This will make using linux a much more pleasant experience.
If you just go ahead and install linux and try to use it the same way you used windows you are almost guaranteed to be frustrated and unhappy. Linux may sorta kinda look like windows but it is not windows and you have to do things a little differently in linux. I am not saying it is hard to use linux just different.
You should go to :
http://distrowatch.com/search.php
and use the search engine to filter which distro would be best for you. You can select a version for beginners, or multimedia, or a special purpose. They are all free to download. You can read reviews and articles about each distro. You can select a distro for ease of use, needs little ram or hard drive space, popularity, most software packages available, amount of customization,etc.
Good luck