I shot a family gathering this weekend and used my D7100 with a SB-700 speedlite. After coming home and researching shooting with flash more I remembered I have a Vivitar 283 (stored without batteries in it). It works good and all and I was thinking about using it as a remote on future shoots. granted it does not offer TTL at all but for adding light it would. how can I sync it with the D7100?
The easiest way is to purchase an optical hot shoe slave. You mount the shoe on a lightstand, mount the flash to the shoe, and use the D7100's flash to trigger it.
The Nikon brand solution is the SU-4 Wireless Remote Slave TTL Flash Controller. It's a nice unit, but with a D7100 you're paying for a film-based TTL capability that you'll probably never use. Instead, I can recommend the Wein HS Hot Shoe slave. At $35, it's less than half the price of the SU-4. Other less expensive units are also available for as little as $10, but I've never used one.
There are two major issues with optical triggering. First, you can't use iTTL, because the pre-flashes will trigger the Vivitar prematurely. So you have to set your on-camera flash to manual or non-ttl automatic as well. Second, the optical sensor can be tripped by other light sources.
The alternatives are using a sync cord or radio transmitters. I'm not a fan of sync cords, but if you wish to use one, I suggest buying a Wein Safe Sync. This will serve two purposes -- it will provide the PC flash connection your D7100 lacks, and it will allow you to safely use your Vivitar 283 "on camera."
Radio triggers are sold with varying capabilities at various prices. I've successfully used the inexpensive units sold on eBay in the past, but as with all too many photographic accessories I ended up "buying twice" and upgrading to iTTL capable remotes.
"There is no real magic in photography, just the sloppy intersection of physics and art." — Kirk Tuck
The Wein Peanut Slave sells for about $14.00 and will trigger Vivitar 285/283 flash units. As stated above you will be limited to setting your on camera SB-700 or built-in Speedlight to Manual flash mode and if you decide to use the SB-700 as a remote together with the Vivitar you must set the SB-700 in SU-4 optical slave mode. For the most consistant results use Manual exposure mode as well.
Also keep in mind that if others will be using flash close by, they will be triggering your remotes if you use optical slaves. To avoid the problem you must use RF slaves.