VSWR measuring, directional coupler
To determine the VSWR you only need to know the magnitude of the reflection coefficient. 

If you know the input power and can measure the returned power, that is also sufficient to determine the VSWR.

Therefore there's no reason a 90 degree directional coupler shouldn't be used in a VSWR measurement.

Edit, to your comment, "I'm asking about 3dB couplers used usually for splitting or combining amplifiers.",

No, these are not appropriate for measuring VSWR because they don't separate the forward and reverse travelling wave effectively. A directional coupler can provide 20 dB or more directivity, meaning less than -20 dB of the forward wave interferes with your measurement of the reverse wave. Since you hopefully have a much less power in the reverse wave than the forward wave, you need this isolation to get a good measurement of the reflection.