I would like to control a SainSmart USB Relay with my Vera3
I’ve never heard of SainSmart before, but in reading their site, I see a couple of probable issues with using them with Vera.
First, their boards appear to use FT245RL USB chips. These chips, if I am not mistaken, appear as parallel ports rather than the more common FT232 or PL2303 USB serial interfaces. Using a parallel interface with Vera will require some custom coding as there is no out of the box parallel port support and I am unaware of any plugins that support it either.
Another thing that I see is this line:
[b]Notice:[/b]When the board is connected to the PC and the PC is restarted, the relays will be toggled several times. This is because of the structure of the FTDI chip.</blockquote>Having the relays toggle several times at restart seems like a really bad idea for many home automation applications. It may not be an issue for lights, but I suspect you are trying to control something other than a light.
Naturally, it depends on your intended application, but I would recommend a relay board the uses a serial(RS232) USB interface and one that does not cycle the relays at power on. I would avoid connecting such a device directly to Vera and would instead use an Arduino to control the relays and Vera can then control the Arduino via HTTP requests or some other TCP/IP protocol.