'Classic' VGA modes use a 31.469 kHz line scan rate, which was chosen to be approximately double the US NTSC video format 15.750 kHz line scan rate. PAL and SECAM video formats have a 15.625 kHz line scan rate. If its on an existing board, odds are the software or firmware configures the MC6845 for NTSC or PAL/SECAM timings, which a VGA or SVGA monitor wont be able to lock to unless its actually a wide range multisync monitor that already supports component and composite video using analog era video formats. If its possible to reprogram the MC6845 for VGA timing, you'll loose up to half your horizontal resolution (depending on the ratio between the original line rate and 640 px/line VGA line rate).
TLDR: Driving a real VGA or SVGA monitor from a MC6845 in an existing system without a frequency doubling scanline buffer in between usually doesn't work, and if it can be reconfigured to work, looses a lot of resolution so may be unusable anyway.