So we should protect people from a coronavirus by vaccinating them against a completely different disease, just because the symptoms are similar?

The most common cause of pneumonia is Streptococcus pneumoniae, which is bacteria, not a virus.  The breathing difficulties as a consequence of COVID-19 is brought about by clotting of the alveoli in the lungs.  The two diseases are quite distinct. There is a significant body of opinion that ivermectin (used traditionally as an anti parasitical) is effective both as a prophilactic anti-viral and as anti inlammatory for post infection treatment.  The World Health Organisation seems to be ignoring the evidence on the ground and Big Pharma is simply not interested because they will make little profit in promoting it (c.f. Pfizer and Moderna raking in  billions of dollars profit on vaccines whilst Astra Zeneca have pledged to supply at cost until the pandemic is over).

Gee, where to start with this? 

A little elementary physiology and virology might be helpful. Pneumonia is an infection which inflames the air sacs in one or both lungs (Wikipedia). Yes, SARS-CoV-2 does that. But its mechanism of cell entry (binding to ACE2, cleaving with the help of TMPRSS2) also works for brain, heart, liver, kidneys, colon, oesophagus, gall bladder, and testes (if you have them). Consequently, these organs are at risk of being infected also, and in many of the severe cases they are. Multi-organ failure is typical in Covid-19 deaths; with most pneumonia it is just your lungs which go out on you (which is enough, of course). Qbit already noted that most pneumonia is bacterial; indeed, one of the worries of proceeding to mechanical ventilation in ICU (for any reason) is a strongly increased chance of bacterial pneumonia. In Covid-19 in particular, that could occur on top of the viral pneumonia from SARS-CoV-2, and is why some of my medically-expert friends got themselves vaccinated against Pneumococcus in early 2020 (I already had mine). Furthermore, even non-severe Covid-19 is not just pneumonia. It is the possibility of myocarditis and pericarditis even in apparently “mild” cases. Likewise unspecified brain involvement. It is the possibility of ME/CFS and other post-acute sequelae which are not at all well understood. It is the possibility of MIS-C (PIMS) in children. 

So, my suggestion for where to start is to inform oneself medically, as above. Next, about the suggestion that a “pneumonia vaccine” might be effective against SARS-CoV-2, the answer is no. As I just mentioned, “pneumonia vaccine” generally refers to a vaccine against Pneumococcus bacteria and there is no earthly reason why that would help against anything except Pneumococcus. 

Third, the suggestion of 70% vaccination rate as a target is, well, an OK idea, although 100% would be a better target. But it avoids the subtleties. For example, it won't do the job against a more infective variant virus than the original one with an R0 of 2.5 to 3. But the question here is: what is a “rate” of vaccination? A rate is something which changes with time. In the case of vaccinations, it can only go up ("having been vaccinated" is a monotone increasing property of populations). Whereas the susceptibility of vaccinated people to disease depends upon the efficacy of the vaccine as well as the general rate at which immunity fades over time. Efficacy is not 100%, although it was found to be close for some of the vaccines against the original Wuhan variant, and the fade is not known at present for SARS-CoV-2. Note also that not all vaccines are equally efficacious. I think you mean to say something like “proportion of those immune through vaccination”, but of course that won't do as so expressed, because immunity is not perfect. People who have been vaccinated can still get the disease (witness Andrew Marr), and one question is then what the transmissibility in this state might be, and to whom. And then of course we still have no idea how long any immunity due to vaccination might last. 6 months? (Now thought to be probably longer.) 9 months? A year? Effective vaccination policy needs to take into account more variables than in your suggestion, and more knowledge than is currently available. All of which is, at the moment, moot, because the savvy medics in this world are  justifiably concentrating just on getting efficacious vaccine into as many arms as possible as fast as possible.

Suggesting that “Big Pharma” is not interested in antivirals for SARS-CoV-2 is just not correct. Regeneron has a cocktail. Eli Lilly has one. Any “Big Pharma” company would love to sell one to those many millions of people who would benefit, if they had one and could produce it in quantity. 

Invermectin may be promising in vitro, but there are few practical studies. Here is one: 

https://www.thelancet.com/journals/eclinm/article/PIIS2589-5370(21)00239-X/fulltext

This is from a couple weeks ago, only involved 45 enrollees, very moderate results. The authors' interpretation: “well tolerated”; nothing about obvious improvement. Anything that looks as if it might work will at some point be looked at in the RECOVERY trial.