As a first step I enabled detect_trace and tested few cases:
First case, default osscore.
Second case, no auto atttach.
Third case, no auto attach and vmixctl attach -M /dev/oss/oss_hdaudio0/pcm0 /dev/oss/oss_hdaudio0/pcmin1 (Though pcmin1 is not a functioning one).
Fourth case, no auto attach and vmixctl attach -M /dev/oss/oss_hdaudio0/pcm0 /dev/oss/oss_hdaudio0/pcmin0.
And for all cases I ran osstest, osstest -V. Results are more or less same except that osstest -V is case of plain "no auto attach" is not working as I thought it was. Atleast not all the time.
Please have a look at the attached dmesg, and see if you can see any think obvious in that log.
For clarity purpose I have run all 4 cases separately and added 2 logs inline and 2 logs below.