It rather depends on points of definition, but it's not generally equivalent - light losses in interferometry are significant, for example, while processing and data reduction is something of a mystery to most astronomers (i've been tangentially involved with a VLTI project and it certainly is to me). On the VLT the interferometers also operate in the infra-red (1-2.4μm for AMBER, and 8 to 13μm for MIDI), I don't know about Keck interferometry as i've not used it.
To give an example of the limitations, the documentation for MIDI says "On the UTs [i.e. the 8-meter telescopes] it is possible to reach an H-band magnitude of 7", which is pretty bright - a 'real' large telescope would go far deeper. But, on the other hand, the documentation says "AMBER is able to resolve features between 2mas (milli-arcsecond) and 50mas with the UTs" whereas NACO, the adaptive-optics assisted near-IR imager on UT4 (i.e. a single telescope), can reach a best resolution around 50mas.
So best to think of optical/IR interferometry as allowing you to achieve the resolving power of a (much) larger telescope under certain, possibly quite severe, restrictions, rather than as a general equivalent