A sua máquina está olhando para o repositório "workstation"? É aí que eu vejo o TigerVNC localizado:
# yum search -v tigervnc | grep -A1 tigervnc
============================ N/S Matched: tigervnc =============================
tigervnc.x86_64 : A TigerVNC remote display system
Repo : rhel-x86_64-workstation-6
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tigervnc-server.x86_64 : A TigerVNC server
Repo : rhel-x86_64-workstation-6
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tigervnc-server-applet.noarch : Java TigerVNC viewer applet for TigerVNC server
Repo : rhel-x86_64-workstation-optional-6
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tigervnc-server-module.x86_64 : TigerVNC module to Xorg
Repo : rhel-x86_64-workstation-optional-6