Yes. The protocol itself is no longer secure, as cracking the initial MS-CHAPv2 authentication can be reduced to the difficulty of cracking a single DES 56-bit key, which with current computers can be brute-forced in a very short time.
The attacker can do a MITM to capture the handshake (and any PPTP traffic after that), do an offline crack of the handshake and derive the RC4 key. Then, the attacker will be able to decrypt and analyse the traffic carried in the PPTP VPN. PPTP does not provide forward secrecy, so just cracking one PPTP session is sufficient to crack all previous PPTP sessions using the same credentials.
Additionally, PPTP provides weak protection to the integrity of the data being tunneled. The RC4 cipher, while providing encryption, does not verify the integrity of the data as it is not an Authenticated Encryption with Associated Data (AEAD) cipher. PPTP is hence vulnerable to bit-flipping attacks, ie. the attacker can modify PPTP packets without possibility of detection. Recently discovered attacks on the RC4 cipher (such as the Royal Holloway attack) make RC4 a bad choice for securing large amounts of transmitted data, and VPNs are a prime candidate for such attacks.
For more information, see http://www.computerworld.com/s/article/9229757/Tools_released_at_Defcon_can_crack_widely_used_PPTP_encryption_in_under_a_day and How can I tell if a PPTP tunnel is secure?.
Issues discovered with RC4 can be found here: http://www.isg.rhul.ac.uk/tls/RC4mustdie.html