A miniature microphone directly on a viola
For live use (or home studio) the 4099 Instrument Microphone for Violin
may well be the ideal solution. This mic has a supercardioid polar pattern that allows the mic to focus on the instrument and get a good isolation from the other sound sources on the stage or bad acoustics.
This also reduces the risk of feedback significantly, while keeping the rich and nuanced sound of the instrument uncolored.
However, because of its high directionality some time should be spend on finding the right placement of the mic. The position will always be a compromise trying to capture the whole instrument with a narrow pick up pattern. The VC4099 Violin Clip included with the
4099 Instrument Microphone for Violin offers a unique solution that is both secure, highly flexible (fits also viola, mandolin & banjo) and very gentle to the instrument.
An even smaller and yet superbly sounding microphone is the 4060 Miniature Omnidirectional Microphone