modify 3rd party machine configuration

In a vendor layer there is a conf/machine/test-machine.conf that
includes some inc files. This inc files have some settings as
MACHINE_FEATURES = "a b c"
IMAGE_CLASSES:append:generic-bsp = " image_populate_tool"
PREFERRED_VERSION_opencv = "2.34"

What I need is to change (add, remove, modify) some of the settings/lines.

How can I do that in MY layer, so not touching the vendor layer.

I thought I am adding a file
my_layer/conf/machine/test-machine.conf (so same name) and put some line
into it like:
MACHINE_FEATURES:remove = "b"
IMAGE_CLASSES:remove:generic-bsp = " image_populate_tool"
PREFFERRED_VERSION_opencv = ""

But that doesn't work and I can't figure out what I am doing wrong.
In bblayer.conf my layer is mentioned later than vendor layer.

How to solve that?

Also I need to supress some files in vendor layer. Is it correct to use
in my_layer/conf/layer.conf lines like:
BBMASK .= " ${BSPDIR}/src/vendor_layer/recipe_test/*" to remove that
from being used and parsed ?

The problem to debug it, until the parsing fails, bitbake -e doesn't
give you proper information about whats going on.

Thanks for any help in advance!
Steve