How much gain would a VHF dipole have?

Sorry Lee26loo...not true. A simple half wave dipole has a gain of about 6 dBi, and as a quarter wave dipole is effectively a half wave, using the ground plane as a mirror, it is about the same, but the polar diagram is slightly inclined upwards, it isn't really suitable for horizontal communications. Use a halfwave on a stick.

dBi means gain over an isotropic antenna, but as an isotropic antenna cannot be made, it is a theoretical concept. An isotropic antenna has a gain of 0 dBi.

yeah Paul is right, it has just above 2dbi because of the doughnut shape radiation pattern, in all directions and it is not a pure sphere hence the higher gain, we call it. If you want to increase the gain in most cases you have to sacrifice the circular coverage. more you focus the energy to an area, more the gain would be. In your amateur radios, yagi antennas are the most popular to increase the gain.

A single element dipole has no gain. The best is close to 1 .

You need to build a Yagi dipole to obtain gain. But it becomes a directional antenna.