Voltage follower with a RF signal
It's not clear exactly what you need, but a few things seem clear. The first is that your 74S124 produces a digital output, so you can use a digital buffer rather than an op amp. Furthermore, your -1.5 volt output occurs as a result of bad physical layout interacting with the sharp fall time of the VCO, and does not represent a guaranteed effect. 

Buffering the VCO with a high-speed buffer such as the 74FCT244 would seem to be your best bet. These are available from sources such as Digikey for a buck apiece. The circuit would look like







simulate this circuit &ndash; Schematic created using CircuitLab
This will not provide perfect amplitude stability, but should probably be adequate for your needs. The output current capacity of the FCT244 is great enough that you should not have problems.