can i use a pci-e 2.0 vga card onto a motherboard with only pci-e (without 2.0)?? will they work just as fine?

Why wouldn't it?

PCI-Express 1.1 has not been saturated yet. We have PCI-Express 2.0 which is underutilized.

Yes.
PCIe 2.0 is fully backwards compatible with PCIe 1.1.

then what's the different between using pcie 2.0 vga card on pcie 2.0  & pcie 1.1 motherboard??

With the current generation of GPU's, there will be little to no difference.
Newer cards may show a larger impact when they need to transfer larger amounts of data across the bus.
 
For some quick benchmarks on the matter, check out Tom's PCIe 2.0 test.
As PCIe 2.0 has double the bandwidth of PCIe 1.1, keep an eye on the 8x PCIe results.
When compared to the 16x results, it will show how much performance you will loose going from PCIe 2.0 to PCIe 1.1.

Just make sure you don't have an old pcie 1.0 motherboard - there are very few out there. That could be a slight limiter on performance.