Hold on a sec. Based on your electrician's repsonse, I think in your statement to the electician you confused two different things. He is right that bad connection would not drop the voltage to 6.7V during cranking. If anything, a poor connection introduces an additional resistance into an electrical circuit and therefore would reduce a load on the battery during cranking, and as a result the voltage drop may be less severe.
Stick with the facts, and be systematic. Fact: voltage drops significantly during cranking. There are a few possible reasons the battery voltage went that low during cranking.
1) your battery was not fully charged because...
...a) you didn't ride long enough so that the bike's charging system can replenish the battery's charge;
...b) you did ride long enough, but your charging system is not working right;
...c) you did attempt to charge the battery, but either did not finish or your charger is not working properly;
2) your battery is defective and is unable to provide enough current during cranking.
3) you have a defective starter which draws too much current which can be caused by...
...a) purely electrical issue in the starter motor windings, for example a partial short between some windings;
...b) a mechanical issue preventing the starter from turning, thus effectively reducing the resistance of the circuit and as a result drawing more current.
So you see, just replacing the battery would accomplish nothing if the problem is caused by any other item above.
In your investigation, you need a positive confirmation for every item you are investigating.
I'd start with the battery. How old is it? If less than a couple of years, it's probably good. If more than five years, it's probably not so good. Fully charge it (let me know how do you know it's fully charged), and try again. Observe the voltage drop, and how the bike cranks. Let us know the results. If you have a friend whose bike starts with no issues, ask him/her to stop by, and jumpstart your bike using their battery while disconnecting your battery from the bike. Do it a few times and alternate: once with your battery, once with the friend's battery, once with your battery, once with the friend's battery, etc. Look for a consistent conclusion - does your friend's battery consistently make a difference?
Regarding the aerosol vaseline, I would prefer a squeesable tube - it's easier to put it where you need it. Not familiar with the spray, but as long as it's designed for this puprose, it will do the job.