If you vote when exhausted the vote will count but give no amount to the post
If you vote a second time, it will remove the first vote, reduce your current voting power by however much you spent on the second vote and the post again will have no amount from you
A third vote will be as if you are voting for the first time (since the 2nd vote actually erased the first vote its as if you had never voted on that post/comment) You will again spend the voting power which will decrease your available voting power and the value of this vote would then be added to the post/comment
Vote 1 - worth $0.00
vote 2 - erases vote 1, if you look at the post it will show as if you had never voted
Vote 3 - say you vote with 100% power and your vote is worth $0.03. The total amount for that post would then increase by $0.03
Probably the best way to answer this question is to refer you to SteemWorld's Voting Power Simulator. https://steemworld.org/@alaisguineasis
The image below shows your Voting Power at 100%, 50% and 2%.
So YES! If you upvote a content even after your VP have dropped significantly (even below 5%), you are still giving a reward to someone although almost negligible. If you look closely at the third pic, when your VP is at 2% you are giving $0.00001 at 100% upvote. This $0.00001 is seen as $0.00 on most front ends but actually it is $0.00001.
It is worth to note though that inside Steem there is what we call "Dust Threshold". All posts or comments with only less than $0.02 potential payout are rounded down to $0.00 on payout(after 7 days). So it is not really recommended to upvote a post or comment if you are only giving less than $0.02 and is the only upvotee of that content.
Your daily allocation is 20% as that what it takes to recharge every 24 hours. You can still carry on voting but not at maximum Voting Power. You still receive curation fees but it all depends on the strength of your vote. If your vote is not very high then you are throwing dust votes that are worthless.