What is the range of frequencies of vibration in a helicopter due to the main rotor alone?
The vibration from the main rotor (and of the tail rotor) are harmonics of the Blade Passing Frequency: number of blades times rotational velocity. This site contains some data on the Blade Passing Frequencies of some helicopter types, for analysis purposes for military helicopters which must comply with MIL-STD-810F.
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MIL-STD-810F, Method 514.5, paragraphs 2.2.6 and 2.3.3 are concerned with the vibration levels that helicopter components and cargo must withstand.
The helicopter vibration environment is a combination of many sinusoidal components due to the main rotor, tail rotor, gearbox, and engine.

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Since the vibrations are also imparted on the surrounding air and then onto the fuselage, acoustic measurements are a good indication of the mechanical vibrations as well. Above graph shows acoustic measurements of seven Hueys flying by, and shows harmonics at intervals of 12 Hz. From the referenced article:

The first notable peak occurs at 24.0 Hz. A series of harmonics occur at 12 Hz intervals thereafter. These are the blade passing frequencies of the main rotor.
The main rotor blade passing frequency is: 5.40 Hz x 2 blades = 10.8 Hz
The Doppler shift increases the apparent blade passing frequency to 12 Hz. The theoretical speed of the helicopters was 76 mph based on this shift.

An interesting factoid is that the highest peak is not caused by the main rotor but by the tail rotor: the one at 123 Hz.
The highest main rotor vibration peak occurs at 60 Hz, five times the main rotor Blade Passing Frequency. There are still noticeable 12 Hz peaks at up to 300 Hz sound frequency = 270 Hz frame vibration frequency.

EDIT

This presentation shows vibration measurements taken off of an unnamed helicopter. The rotor frequency is detectable as a small peak, the blade frequencies are much more dominant.