A bit of your problem is not much of us use composite inputs these days. So there is not much help for your specific situation, of people that use the same cards.
You can experiment with other composite input cards, or get multiple or a multi-input SDI card with composite to sdi converter(s).
That also opens up other possibilities.

For your information:

I've got 4 SD camera's (Sony SNC-RZ) connected to an Osprey 440 capture card and got 300ms delay between camera activity and full screen output.
Using another graphics card has no effect on the latency (GT610 / Radeon HD7750 / GTX750Ti).
The composite output of the Sony SNC-RZ camera's and the preview screen of the Osprey have no visible latency.
The vMix render time is about 2ms and CPU is max. 15%. (no recording and no streaming)

My system can be the bottleneck: Win7 32bit, 4GB ram E8600 Core2Duo (3,3GHz).

I did a FFMPEG stream test with a fast GPU and a fast CPU:
My E8600 Core2Duo with Geforce GTX750ti and harware encoding enabled has a much better quality stream result than
an i7 4560U with Intel HD 5000 graphics without hardware encoding (total CPU lower than the E8600).

Congratulations ;)

Two things, First right click on vMix icon and select Run with Graphics Processor > High performance Nvidia processor.
Next after vMix is open go to settings> performance and select check box enable Optimus graphics compatibility.
restart vMix