Assuming you're buying a new flash, here's what you'll be missing.
Decent build quality and reliability. Vivitar no longer makes flashes, they license out the name to Sakar, and the Sakar units are not the old reliable workhorse units, although they look and are named the same. A brand new 285HV is not like a vintage 285, and is very much the same as a Cactus KF36.
1/8 manual power setting. You'll have 1, 1/2, 1/4, and 1/16. But no 1/8.
A standard sync port. The Vivitar sync port is non-standard--it's not a 3.5mm minijack and it's not a PC port.
TTL communication and all the attendant goodies.
However, you will gain an autothyristor.
- TTL communication and all the attendant goodies.
However, you will be gaining a 3.5mm sync port, and built-in optical slave modes. And the flashes are manufactured and tested to known high standards.
The 580EXII gained features over the 580EX: menu commandability (with Digic 4 and later bodies), weather-sealing, an authyristor mode, and a PC sync port being some of the main highlights. So, yes, there are reasons to go for a 580EXII over a 580EX.
Manual vs. TTL
TTL communication isn't just about having e-TTL, it's also about having the camera and flash communicate with each other. You need this for features like e-TTL, high-speed sync, and changing flash settings from the camera menu. And most TTL-capable speedlights with rotating heads can be put into Manual mode.
eTTL on the flash is like having Av mode on the camera. It's a way to automatically set the flash power level based upon through-the-lens (TTL) metering. The camera tells the flash to output a "preburst" flash of a known brightness level, meters it, and then adjusts the flash's output power for the main burst, based upon the reading and the flash's power limits. It's fast and it's convenient, and is most typically used for on-camera run'n'gun event situations (parties, weddings) when you may only have one chance to get the shot, and you're moving through changing lighting situations.
High-speed sync is the ability to use a shutter speed faster than your camera body's maximum sync speed (usually 1/200s).
Menu commandability is a nice luxury when shooting with an on-camera flash, but today there are a lot of affordable radio triggers (Yongnuo YN-622, Phottix Odin, Pixel King) that use this menu command protocol between the camera and the flash to give you control over a remote flash from the camera body via the flash control menu. This is extremely handy when it comes to adjusting manual power levels, particularly if your flash is placed somewhere hard to reach (or has its body inside a softbox).
To my mind, unless you know you're going to do studio lighting setups exclusively, for a first or only flash, a TTL-capable unit (preferably an OEM one that won't be subject to the vagaries of reverse engineering) is best, because you can use it with equal facility for on-camera run'n'gun shooting with eTTL, and off-camera Strobist setups. The manual-only flashes are typically better as 2nd, 3rd, 4th, etc. units when you get into off-camera flash.