Why This Analyst Downgraded Vonage Without PT?

Oppenheimer analyst Timothy Horan downgraded Vonage Holdings Corp VG to Perform from Outperform without a price target.
Horan cites valuation for the downgrade, with the shares up 28% year-to-date.
On a potential buyout, the analyst says Vonage is "unattractive to strategics" and that private equity is unlikely to pay much more than $18 per share.
Horan also believes the company will face competitive pressures from DialPad, OOMA, and WhatsApp.
He expects enterprises to downsize unified communications as a service due to low utilization.
Price Action: VG shares traded lower by 2.11% at $15.8 in the premarket session on the last check Wednesday.