Each purchase is insanely expensive for what it offers you. For example, to get an additional 0.1 votes per second, you might end up paying 17000. That's bonkers, but there are a lot of seconds in 24 hours (86,400) and consecutive playthroughs provide multipliers. So, it can be tough to know if you should buy another level of leaflets for maximum benefit, or just stop wasting votes that won't be earned back.
The cheapest items don't earn much back, but are easy to acquire. The most expensive items are massive earners, but you have to already have a strong vote-acquisition infrastructure, which takes time. During that time (in fact, every single second), the long-term value of those votes decreases.

Then, we must ask ourselves to solve this math problem:
Is the Cost (C) less than or equal to the Vote Total gained(Vt) times the Multiplier (M), given how much Time is left (T)?
C > Vt x M x T x 60 seconds/minute x 60 minutes/hour

If you have a multiplier of 1, the following table tells you how much you can afford to pay to break even by the time the campaign is over. In reality, it's not a huge deal to buy 1 extra or 1 too few, since usually it will ALMOST be the same as if you had purchased the perfect amount.

To determine whether you should make the purchase, figure out how much time you've got left in the campaign. Find that row in the table, then move to the column of the item you plan to buy. Multiply the value by your multiplier for this campaign, and buy until the cost reaches something greater than that value.

The exact strategy for maximizing how fast your VPS goes up isn't addressed here, only the point at which you should no longer spend votes on each category of upgrades.

Of note, with a multiplier of 1, the Endorsements are NEVER worth buying, and with a multiplier of 2, they are only worth buying the first level if you grab it in the first 4 hours, which is HIGHLY unlikely.

The spreadsheet I have does a lot more of the work for me. I fill out the start date/time and multiplier for the current campaign, then I continually update the minimum cost of something that hasn't already past the worthwhile buy period. The spreadsheet then turns the column green if it's still a worthwhile buy, or red if it's too late to bother paying so much. It is also accurate to the second, rather than the hour.

You can download it here:
{LINK REMOVED}http://www.mediafire.com/file/dcu0uk8skklax6c/Campaign_Clicker.xlsx

Thanks to Skeletor for the suggestion of how to share it.

To use this spreadsheet, fill in the three orange boxes.
The first is the date and time that the campaign started. An example of the format would be: 11/8/2016 5:00:00 PM (depending on your timezone, the campaigns always start/end at the same time every day across the whole world).
The second is the multiplier (starts out as 1).
The third is the optional box where you can enter a cost value in order to highlight boxes red that are no longer going to pay off before the end of the campaign. I use it as the "current minimum", as in the lowest cost of anything that still might pay out.

You might decide to invest in upgrades in order to get more coins. This helps because coins affect your multiplier, so you can consider it a long term investment.

The following is according to this thread:
https://steamhost.cn/steamcommunity_com/app/485650/discussions/0/341537388318190339/

You earn 1 coin for each upgrade unlocked.
You earn an additional coin at 10 for each upgrade.
You earn an additional coin for 100 for each upgrade.

Because of this, it may be worthwhile to get a couple extra levels to reach lvl 10 or 100, even if you don't earn the votes back 100%.