sudo mount -t ext4 -o loop,rw,noexec,noatime vendor.img /mnt/vendor
sudo dd if=./vendor.img bs=1MiB of=./vendor.img conv=notrunc oflag=append count=5 #to add 5MB
sudo losetup | grep vendor #get the right loop device, in my case loop25
sudo losetup -c /dev/loop25
sudo resize2fs /dev/loop25
sudo vim /mnt/vendor/etc/fstab.mt6785 #removed the avb options in vendor, system and product
sudo umount /mnt/vendor
now i have an edited (bigger) vendor.img in the A slot:
$ du ./super*/*
1552656 ./superA/product.img