Is there any definition of life which makes viruses undeterminable?
Your last sentence is the key: defining life really is just finding a dictionary definition that we can agree upon. Biology is something that defies discrete definitions at times: "What is it to be alive?" "What is a species?" maybe even "What is the wild type allele of a gene?"
I would recommend not looking at viruses as a challenge to determine if they are alive or not so much as an excellent opportunity to discuss what we think are important characteristics of life.
Life can alternately be described as:
"Comprised of self-replicating cells" (a paraphrase of the "Cell Theory of Life" 
or
As things that embody at least most of the following characteristics:
1. Self-Replicating
2. Metabolizing
3. Growing
4. Showing signs of adaptation
5. Being organized
6. Respond to their environment
7. Being comprised of cells

I like to think that we should focus on extraterrestrial forms when we define life. i.e. what would we want to see in an extraterrestrial in order to call it 'life'? While some are troubled by calling viruses alive here on earth, the same people might be willing to say that we have found extraterrestrial life on another planet if it was similar (granted, it's hard to imagine this kind of life existing without a host...)

as an aside: You might also ask whether this question is fit for this stack as it can not be supported by literature references (at least none that would actually support a conclusion). So should this be posted as 'Biology' or 'Philosophy'?