you don't need to disable vpu, if you need use software decoding, you can use libav(open source), but I don't know why you use software deocoding since imx8M has hardware decoding?
for how to install the libav, pls refer to the steps as below:
The following steps show how to install gstreamer1.0-libav into a rootfs image.
1. Add the following lines into the configuration file conf/local.conf.
IMAGE_INSTALL_append = " gstreamer1.0-libav"
LICENSE_FLAGS_WHITELIST = "commercial"
2. Build gstreamer1.0-libav.
$ bitbake gstreamer1.0-libav
3. Build the rootfs image.
$ bitbake
$ <image_name>