Esperanto Language Stack Exchange is a question and answer site for teachers and students of the Esperanto language. It only takes a minute to sign up.
Sign up to join this community
For example, I've seen the word dumtempe before used to mean for a while or for some time.
I used the word today, then started second-guessing myself and looked it up at Vortaro.net, but it showed no results. Isn't Vortaro.net basically an online version of PIV?
Was I wrong to use the word? Have I been led astray concerning the word?
This has happened multiple times before. 
Remember that a dictionary is essentially a tool to help you understand a word. You don't have to not use a word because it is not in a dictionary. PIV, while influential, is just a dictionary among others, and is regularly updated in order to reflect the language usage. People should be able to speak Esperanto without having to use a dictionary everytime. I don't remember the last time I used a dictionary in French. If you understood a new word without the help of a dictionary, and the word is not suspicious, then it should be safe to reuse the word without checking a dictionary.
vortaro.net is exactly what you say, an online version of PIV, so it's the most comprehensive dictionary of Esperanto that exists. It has its problems and its critics, not without good reason, but it's a valuable resource for everyone. I use it regularly, both online and on paper.
Your example is a compound word, and PIV can't possibly list every compound word that it's possible to create in Esperanto. That said, it does include many commonly used compounds, and it turns out that that includes "dumtempa", complete with a definition.
So should you stop using vortaro.net? That's your decision, but I don't understand why you would.
My personal opinion would be that it's alright, as you have not introduced any new roots, but just combined two existing ones. As long as the meaning is clear, including for people with other native languages, that should be perfectly fine.
Esperanto can be productive in combining (existing) morphemes, and they cannot all be listed in a dictionary.
No, dumtempe is a good and useful word.
Through the use of affixes and word compounding, there are almost unlimited word combinations (as long as the compounded word makes sense.) PIV/Vortaro.net is a great tool to find official, in-use words but doesn't list everything.
I would suggest adding ESPDIC to your list of tools (http://www.denisowski.org/Esperanto/ESPDIC/espdic_readme.htm) it has more than 63,000 words and contains many different types of words. But be wary, don't use it as your go to dictionary; it also contains words that are seldom used and, in some cases, just wrong.
A quick search at ESPDIC reveals:
"dumtempe : for the time being, meanwhile"
Nothing wrong with dumtempe It comes up 8 times in the Tekstaro, including in such works as Vojaĝimpresoj, Vespera ruĝo anoncas ventegon, La Kiso, Artikoloj el Monato, La Ondo de Esperanto, and Gerda Malaperis!
