Likely the tachometers are alternator-driven. A replacement alternator may not have the correct # of poles and/or pulley. The tachometers may have an adjustment or selector switch on the back, if not, you will have to replace the alternator(s). The VDO instrument most likely is installed with its own sensors and if so should be accurate when it works. Intermittent operation is likely a connections issue.
On Amazon.ca you can find an optical tachometer for $40, shipped. It’s all you need.