The design decision to use an acoustic suspension enclosure or a ported one is completely independent of the voicing of the high frequencies. So, attributing certain high frequency characteristics to sealed enclosures or vented ones is incorrect. An acoustic suspension enclosure, properly designed, and mounted in the same volume enclosure as a ported one, will roll off more slowly below its resonance frequency, produce less harmonic distortion, or "doubling", and is less likely to have the characteristic peak or hump in frequency response near its resonance frequency. On the other hand, acoustic suspension enclosures tend to be less efficient than their vented counterparts. Vandersteen speakers certainly have bass enclosures which I believe are transmission lines. This is basically a vented design. There are excellent designs of either type and I wouldn't rule out any speaker simply based on its enclosure design.
