As I said, 100% is the aim, if not a practical possibility. The effect on the amount people are prepared to pay to buy a piece of land is the same. Zero, or close to it.
Vacancies act to regulate the market. This being one instrument to set rents, thus maximise the income from land. So at any one time there will be a range of rental values collected (whether by private landowners or the state) from 90% to over 100%.
As it happens, with the current plethora of data we have now on rents, selling prices and vacancies its very easy to tax 90-95% of land rents. With dedicated computer modelling easy to get better than this should we wish. Not a problem either way.
As for valuations, housing (which is by far the biggest part of agg land values) is the most straightforward, simply due to the fact homes can be put into classifications and compared like for like between locations.
Without any fancy computer modelling, this would capture > 90% of housing land rents. Of course, if the aim was to collect as much land rent as possible for tax revenue, it would be worth investing in computer modelling which would get very close to 99%.
Commercial property is a little more difficult, but the same principles apply.
As a thought experiment. Imagine you owned all the land in which ever country you lived in and wanted to maximise your income from it. I think you could do better than 2/3rds given the amounts at stake.
The aim is to collect 100% of lands rental value, which drops its selling price to zero. So people would assess the value of land as zero if they had to pay a full LVT.