I have a VX-5R also and at one time I wanted to do what you are suggesting here. The CT-44 doesn't provide any isolation, it's just a breakout for the odd 4 ring Yaesu connector, so be mindful of that. If your soundcard voltage is high it can damage the 5R.
The CT-44 doesn't enable any sort of vox capability, so just having audio running through it won't cause the HT to key up. Check page 11 of the manual for the required resistance to open the PTT circuit.
Best bet, make or buy an interface which will have all the necessary bits (isolation, PTT activation, cables). There are several places to buy them online. Just be careful when asking questions of the sellers, some don't even like to bother if they know it's for an HT "because of the duty cycles involved." Personally, I believe if I fry my gear, it's nobody's fault but my own!
Hope this helps some
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