You could just try this out an see for yourself. Most motherboard chipsets do not allow full PCIe bandwidth to dual GPUs. The 16x speeds will normally gimp back to 8x per card. This maybe be enough it might not, you would have to test this. Remember any PCIe speeds below 3.0 you have to account for about 25% overhead. You generally want to optimize the bandwidth paths between Memory, CPU and GPU to be as high as possible.

Vegas will only use one card for timeline GPU playback, so install the 2nd card with no monitors connected, and configure Vegas preferences to use the dedicated 2nd card. This would allow all resources on that card to be used for calculations, and not compete for GPU time with windows GUI or other applications. Use AMD System Monitor or GPU-z to determine, if the correct card is being used as Vegas may list the cards with identical chip names. Restart Vegas each time you select a different GPU to test.

Crossfire will not benefit Vegas. Only certain applications and games will benefit from Crossfire.

Most likely you will find that your Power supply will not have the capacity to handle the 2nd card, of have the connectors to use it.