Convert VSDX to BMP

Microsoft Visio supports export to JPEG format, so getting a BMP file from the JPEG should not be a problem with any graphics editor, even default Windows Paint program that allows you to resave basic graphics just fine. 

A viable alternative yo can try, would be to use some other program with Visio format support, but it's usually better to use the native program as 3rd party program may have problem with latest proprietary and closed formats, such as VSDX.