In any case, what you probably need is a better antenna, rather than a preamp. Either a better all-band antenna or even a tuned antenna to the frequency of the VHF-Hi band portables you are trying to monitor. Find out the exact frequency and get an antenna that is at or near that frequency. E.g., if the frequency you want is 154.8300, then you can get a VHF antenna that covers 140-170 MHz or an antenna tuned to about 154.5 MHz or even to the exact frequency. Use this antenna when your are trying to monitor those elusive VHF portables you are trying to pick up. Use the all-bander antenna for general use.
You will find that a good monitor (scanning enthusiast) has an arsenal of all-band, dual-band, tri-band, single-band, and fine-tuned antennas.
If you are at home trying to monitor, then a basic base antenna on the roof or the side of the house will make a huge difference. The same thing if you are mobile in a vehicle, a basic trunk or roof mount antenna will make a huge difference, as opposed to using the rubber duckie.
First of all I go with Dave's advice, 80% of your scanner is on the roof, 15% is in between and 5% is in the shack. Use an amp as a VERY last resort, more often they cause more problems than they solve and are less than useless in an RF rich environment.
Last but not least car to car simplex is difficult at best unless the units are in your immediate vicinity. Cars with high powered transmitters are one thing but 5W VHF and 2W UHF portables are good for a mile on average and a mile is but 5280 feet. Improvement never hurt anybody but don't set your sights unrealistically high, a fall from above hurts worse than tripping over a stone.