If the VLT (very large telescope) can use interferomerty to act as a 200 meter mirror, then why are they trying to build the 39 meter E-ELT? What advantage does the E-ELT have, if the VLT can act as a 200 meter mirror?
Ok, not sure where you get 200m from, the maximum baseline for the VLT interferometer is 130m. Nevertheless, your question still stands.
There are a number of reasons. Firstly, it is important to understand that with relatively limited number of baselines (4 VLTs = 6 baselines) one does not get actual images out of the interferometer, but rather "visibilities" that enable one to work out whether sources are double, extended or whatever. Second, the interferometry only works at infrared wavelengths. This is a technical issue that might be solved in the next decade or so, but a general rule of thumb is that the longer the baseline, the longer the wavelengths you will be limited to use. Thirdly, the light gathering power is limited to the light that can be gathered by the constituent telescopes. I think this also limits the kinds of instruments you can use since the required delay lines and optical components of the system make everything inefficient. Fourth, there are limitations of the abilities of optical/IR interferometers caused by turbulence in the atmosphere. This limits them to very small fields of view and the performance does not reach the ideal case. It also means that you are limited to bright objects because individual exposure times need to be very short.
The E-ELT is designed to work at optical and infrared wavelengths and will gather enormous amounts of light. Not everything in astronony needs spatial resolution; sometimes what you need is just a bigger light bucket. Examples include: deep imaging of the universe, gathering spectra from faint objects; accumulating enough photons to do transmission spectroscopy using exoplanet transits.
It's horses for courses really. At optical and IR wavelengths the future will likely be big telescopes in space and even bigger interferometers in space.
The quantity of light gathered is the important difference.
The VLT and its kind can use interferometric methods to achieve the resolution of a much higher telescope - that allows detection of fine detail that previously couldn't be seen.
Large single aperture scopes like the E-ELT don't offer such a high resolution, but they collect a larger quantity of light so can produce acceptable images of much fainter objects. Part of the E-ELT's core goals is imaging of very distant galaxies, and for this it doesn't need exceptional resolution, but it needs very sensitive light gathering. ("Light" including part of the infra-red region too, though that's separate question.)