What year is your Z? Spring compressors are absolutely needed on the first generation Z's, and I suspect they're needed on later cars as well. There's a lot of tension built up in those springs, trying to take them off without using spring compressors is very dangerous. You can get a set for under $20.
you do need a spring compressor, not necessarily the vice. just get someone else to hold the assembly. you can go to autozone and get "loan-a-tool" spring compressor, with a deposit, then when you're done, you take them back and they give you all your money back, they got lots of special tools for loan.
some guyz have said that they were able to change their struts with no problems. mine took slightly longer to change than anticipated. you DO need spring compressors. the job would be near impossible with them. the cheapest pair i could finder were $50, but i didn't look hard enough to find them cheaper. i was able to rent them for less than $20. obviously, you need the ones that clamp on the outside of the spring. it might help to disconnect the brake lines in the rear. good luck.