You are describing the difference between VAMOS-I/SAIC and the VAMOS-II. The latter is able to effectively suppress the AQPSK interferer.
VAMOS level 1 MS is just a SAIC MS which is able to use the TSC Set 2. There is no special interference algorithms based on AQPSK usage. Therefore, their “SCPIR thresholds” are the same.
It can be seen in the 3GPP “MUROS” tech spec. The link level simulations show no difference between SAIC and VAMOS-I, as far as I remember. They both are described as “DARP”, with no further difference.
VAMOS-II on the other hand is much more efficient and can work with SCPIR as low as -10dB !
For SAIC/VAMOS- I, the SCPIR must be greater than -4dB.